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My planet has interesting geography; there are large islands, each with a unique civilisation of people. But separating them are impassible barriers; they consist of a double range of tall, steep mountains, and between them a sea of "dead water".

Top view:

Incompetent artist's impression

Side view:

side view

These barriers were put into place to separate the islands and their civilisations, until the invention of flight. The mountains are smooth and steep, and made of hard rock; so very hard to scale (they are 3 to 5 km high). And once you get to the other side, you have a big, wide sea, of dead water. If it were normal water it would take several days to cross (them a couple hundred km wide) but they are filled with dead water; which is a substance that is made to be really, really hard to pass on a boat. It has the following properties:

  • It is toxic to every known form of life. It doesn't necessarily have to hurt the skin, but ingesting it is (eventually) fatal.
  • It is chemically stable over a time period of tens of thousands of years. Millions of years is not required; this was made by gods in order to separate their test subject civilisations. The first civilisation to cross the barrier wins!
  • The stability requirement also entails that it either does not evaporate under regular weather conditions, or it does so by breaking down somehow into inert elements. What I want to prevent is clouds of this dead water forming and raining down on the surrounding islands, thus spreading its toxicity and killing off all life. It should remain confined to the "canals".
  • Furthermore, it should somehow survive rainwater, which would land in these canals but be unable to leave as the mountains are in the way. Tall mountains do produce a rain shadow but we're talking about tens of thousands of years here; I imagine that that would accumulate over time.

Is there any real-life chemical substance that has all these properties? If not, how close can I get?

Also note that the experiment setting entails that real laws of physics govern every aspect of the planet. The materials do not have to be naturally occurring in the quantities I require, but they should be able to survive on their own, without any outside intervention whatsoever.

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    $\begingroup$ Even without acid, this kind of sea between mountains would be hard to go trough, simply because the ship can't get trough the mountains. You either need to lift boat in the mountains, or build a new one on the other side. And as you describe this mountains, it will be a hard task $\endgroup$ – Kepotx Aug 27 at 9:15
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    $\begingroup$ How wide are the canals of dead water ? One mile, ten miles 100 miles, 1000 miles? Also how tall are the mountains - Everest size? Is the planet Earth-like in its gravity and atmosphere? $\endgroup$ – chasly - supports Monica Aug 27 at 10:13
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    $\begingroup$ @chasly-reinstateMonica the description for the gods tag goes: "For questions about gods, the creation of gods, their characteristics, and the effects they have on worlds." That is not applicable to this world; since after the initial creation, the gods will desire to have zero continuous effects on the place lest their interference ruin the experiment. earth-like is appropriate, thanks for that. $\endgroup$ – KeizerHarm Aug 27 at 11:57
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    $\begingroup$ @Kepotx just pull the boat over the mountain youtu.be/TRf66nblmxw $\endgroup$ – Tom Sol Aug 27 at 14:23
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    $\begingroup$ @RobbieGoodwin you continue to perplex me. The gods are not influencing this system for the duration of the experiment, that is why they are not important. I'm sorry if you got confused, but the 21 answers all have no problem with the mention of gods, so I will not edit them out of the question, nor will I make them more important than they are. $\endgroup$ – KeizerHarm Aug 29 at 7:20

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dilute radium-226 bromide solution.

The radium (half-life 1600 years) would decay into radon-222, a radioactive gas with a half-life of 3 days. The water would be highly radioactive, boiling hot and bubbling radioactive, toxic and corrosive gasses. the gasses would decay quickly and not make it out of the 'dead canals'. It would also generate bromine, helium and hydrogen bromide. All of these are either inert or would react/dissolve in water and into the 'dead sea'.

The RaBr2 would have to be dilute enough that it wouldn't boil off all the water before sublimating itself.

The only way to get over this would be to fly above the radon clouds in a radiation shielded plane.

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    $\begingroup$ I accept this answer for creativity, for sticking to the sea concept, and for being not just lethal, but lethal in a variety of ways. Perhaps a wooden boat would survive this but there's nothing a pre-flight civilisation can build that would shield against radiation, so this answer works best for me. $\endgroup$ – KeizerHarm Aug 28 at 20:24
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    $\begingroup$ I see three main problems with this solution: 1) the 1600 year half-life isn't really compatible with "stable over a time period of tens of thousands of years", 2) the radium will anyway escape over time, most likely by evaporating along with the boiling water (the boiling does distill the water somewhat, but no distillation is 100% perfect), and 3) the radium vapor would then rain out as radioactive fallout over the surrounding areas. The first two issues could be mitigated if there was some source of additional radium over time, but that would still leave the problem of radioactive rain. $\endgroup$ – Ilmari Karonen Aug 29 at 11:54
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    $\begingroup$ @KeizerHarm Radiation shielding is dead simple even for a primitive society. All you need is the ability to mine and refine lead, and that's not all that hard to do (humans did it in real life before we even had written language). The issue here is radiation shielding something that floats and completely protecting against the corrosive gasses. $\endgroup$ – Austin Hemmelgarn Aug 29 at 22:50
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    $\begingroup$ A half life of 1600y, wouldn't this mean that after a few thousands year the lethality would sink drastically? On the scale of a planet, that is very short $\endgroup$ – 3C273 Aug 30 at 4:23
  • $\begingroup$ because of just how lethal radium is, even reducing the radium to a fraction of the amount would still be extremely lethal. the amount of escaping radium bromide/radium would be negligible as the mountains (as stated in the question) prevent most of the vapors from leaving the channels. $\endgroup$ – WhoCares Aug 31 at 13:32
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Acid.

crater lake at el chicon

Crater lake at El Chichon

https://www.usgs.gov/faqs/can-lakes-near-volcanoes-become-acidic-enough-be-dangerous-people-and-animals?qt-news_science_products=0#

Crater lakes atop volcanoes are typically the most acid, with pH values as low as 0.1 (very strong acid). Normal lake waters, in contrast, have relatively neutral pH values near 7.0.

The crater lake at El Chichon volcano in Mexico had a pH of 0.5 in 1983 and Mount Pinatubo's crater lake had a pH of 1.9 in 1992. The acid waters of these lakes are capable of causing burns to human skin but are unlikely to dissolve metal quickly. Gases from magma that dissolve in lake water to form such acidic brews include carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen chloride, and hydrogen fluoride.

The water from an acid lake might evaporate but the ions that make it acidic will not. The acid will not go somewhere else. pH 0.1 is enough to sort out most stuff in contact for any length of time. Volcanic lakes like this can also give off fumes that can irritate eyes and lungs and even exclude oxygen, so you might asphyxiate before you got close enough to dissolve in the acid.

The acid can regenerate because of volcanic gases upwelling from beneath.

Best: lakes like this are real! That deep weird green is not from algae, but dissolved iron, sulfur and other ions.

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    $\begingroup$ Acid's will dissolve lots of things, but it's a slow process, it may dissolve your boat completely in a few weeks, and it'll stink, maybe in a few hours, but you'll be able to get across it. There are videos of people doing tests with strong acids online; Eg "I phone 7 vs world strongest acid" - youtube.com/watch?v=N3xyICmQHy8 (It survives, turns on, but is etched), or "battery vs acid" (24 hours later it still looks like a battery) youtube.com/watch?v=hQsOS08fvss $\endgroup$ – Ash Aug 27 at 11:38
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    $\begingroup$ @Ash that's still going to give you serious issues with the toxic fumes, and with the fact that any ship that you want to use to make the trip is going to have to come over or through the mountain with you and then cross the dead-water sea without taking holes. "No longer seaworthy" happens a long time before "fully dissolved", and the dead seas are apparently quite wide. $\endgroup$ – Ben Barden Aug 27 at 13:12
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    $\begingroup$ @BenBarden also, for civilization to even attempt it, it would have to be reasonably certain of what is on the other side. $\endgroup$ – Gnudiff Aug 27 at 17:26
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    $\begingroup$ @Gnudiff Sometimes humans do stuff just because it's hard. First to orbit? To Moon? To Mars? All check with permanent habitation well underway. Shave off 0.03s of the world record? Cool. Mathematically prove that it's impossible to shave another 0.03? Even better. Run 40 kilometers through hot sand? Been done. 3 kilometres of smooth rock is nothing compared to that, and that's before somebody carves out the stairs. $\endgroup$ – John Dvorak Aug 27 at 18:16
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    $\begingroup$ @JohnDvorak 3-5km ir just mountain, we are taking about the wide acid sea, several days to cross. $\endgroup$ – Gnudiff Aug 27 at 18:19
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Dead water is a misdirection!

enter image description here

Why not Acid?

Someone else suggested acid - of the stable liquids I believe this is the best choice, however:

  • glass was being manufactured around 100AD, and a small glass canoe could survive the river of acid (Yeah you gotta hike it over a mountain, but they'll find a way, same way they got monastery windows up into the mountains).
  • Some plastics could also survive it, and a rush towards petrochemical refinement could lead to a plastic boat crossing the acid without their peoples discovering flight.
  • Or someone could just bulk mine and then truck in lime, or calcium carbonate, or some other common alkalies.
  • It also may dilute over centuries of being rained on, and that dilution could be sped up by an industrious people pumping sea water in. It may also overflow if too much is created, killing the neighbouring zones.
  • Acid takes a while to eat through things. A thick hull could get through.

So what then?

Another answer pointed out there is no liquid that can do it. I believe that it correct. So don't use a liquid. Why not use something deadly to humans, but life giving to the bulk of the planet, and found in bulk on many planets in our solar system?

The "dead water" is a myth, meant to misdirect the uniformed who try to cross it and die in the process. The side of the mountains facing the "dead water" are solid stone, unsuitable for trees or plant life to sink their roots into, leading to the "dead" description of the valley, but also crucial in keeping it that way.

Frozen dry ice at the planets poles (eg like on Mars) slowly thaws over the milinea, with large amounts of Carbon Dioxide trapped in the valleys between the high cliffs. When the CO2 overflows the valleys into the populated parts of the planet (as it would seasonally or during extreme weather), the trees and bushes suck it up and pump out extra oxygen into the habitable zone.

You could also have the summer pole thawing CO2, and the winter pole freezing it, that way the CO2 is always flowing. So any effort to pump it out or displace it (or just rush it after a big storm) wont work.

No boat can conquer the enigma of the dead water, because the water isn't dead, it's the air above it.

Crossing the dead zone without flight would require the discovery of CO2 (1750) and the production and availability of pressurised air canisters. The first reference I can find to them being available is 1887, within 15 years of flight being discovered, but they weren't mass produced until WW1.

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    $\begingroup$ Make your acid HF and it would even dissolve a glass canoe. And the fumes are really bad. Also glass is dense, brittle, and hard to make in big single pieces. Making a completely glass boat that could withstand being hauled up a steep smooth mountain and lowered down again wouldn't be as easy as you imply. However acid this strong would attack the rock (there's a lot of silicate in rock that would suffer from HCl or HF). $\endgroup$ – Chris H Aug 27 at 10:26
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    $\begingroup$ ... Fluorite (CaF2) might be a good material for the dead side of the mountains - I don't think it would react with HF, and it could be rather smooth, making climbing down treacherous. It could even keep the HF topped up, especially if there's a source of sulphuric acid as that's the reaction used to make HF $\endgroup$ – Chris H Aug 27 at 10:29
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    $\begingroup$ People are really good at detecting CO2 in the air that they breathe, to the point that we mix it into halon systems in order to keep people from suffocating in the halon. They'd notice near-instantly that it was bad air rather than dead water. $\endgroup$ – Ben Barden Aug 27 at 13:16
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    $\begingroup$ @BenBarden You sure? epa.gov/snap/carbon-dioxide-fire-suppressant-examining-risks "possibility of carbon dioxide leaking or flowing into adjacent, low-lying spaces such as pits, tunnels, and passageways. In these cases, carbon dioxide can inadvertently create suffocating atmospheres that are neither visible nor detectable." $\endgroup$ – Ash Aug 27 at 13:31
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    $\begingroup$ Yes, I'm sure. dhs.wisconsin.gov/chemical/carbondioxide.htm - under the health effects tab. People start complaining about poor air well before it hits anything lethal. $\endgroup$ – Ben Barden Aug 27 at 13:42
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Heavy-ish crude oil (not heavier than water).

  • toxic for almost everything.
  • slow evaporation and heavy vapors that don't readily jump over the mountains (and if they do, the worst is the smell)
  • stable for the timespan required
  • rainwater simply sinks into the water body below. Water bodies can even be connected underground with no major ill effects.

Bonus: makes the hard mountain climbing even harder once you get oily.

Bonus: it may even be useful at some point.

Bonus: Swiming in oil is hard or probably impossible, human body doesn't have enough buoyancy and the higher viscosity doesn't help either. Boats need to be overengineered for both stability and buoyancy, or else they also sink.

Oil surface doesn't evaporate water so there are no thunderstorms (ignition source) above it. The high mountains get all the lightings at a safe elevation where there is no oil to ignite. Fire over open oil surface is just as unstable as in the real world and winds quickly kill it. In a still conditions the valey fills with CO2 so no fire either.

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    $\begingroup$ And also, you left out “highly inflammable” from your list of properties. One lightning strike and your sea (and everything nearby) is gone. $\endgroup$ – Mike Scott Aug 27 at 7:17
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    $\begingroup$ @MikeScott Sorry. Petrol in a lot of languages just means crude oil. I meant crude oil. And some sorts are really hard to ignite over open surface. Doubly so if the sea level for the oil is higher (it will be, if there is a hydrostatic equilibrium between the seas). We can think about some mechanism to make the fires local. $\endgroup$ – fraxinus Aug 27 at 7:27
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    $\begingroup$ @MikeScott No need to have it be naturally occurring: the gods waved their hands and the seas were created. It's just got to be stable in natural conditions, without any further intervention. $\endgroup$ – KeizerHarm Aug 27 at 7:59
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    $\begingroup$ thunderstorms move, it doesn't matter if there is no evaporating water nearby. $\endgroup$ – John Aug 27 at 16:56
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    $\begingroup$ Note that crude oil that is exposed to the air for a long time tends to turn into "tar" which is perhaps an even better barrier. See en.wikipedia.org/wiki/Tar_pit $\endgroup$ – plasticinsect Aug 27 at 23:31
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Forget chemical. A sea full of radioactive salt is effectively impassable to a pre-flight civilisation, not affected by rain, toxic to all life and will last 40,000 years with judicious choice of isotopes.

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  • $\begingroup$ Are there isotopes that give out radiation for that amount of time, always in such a strength that it's legal in a day? I thought that the more powerful radiation, the faster it decays. $\endgroup$ – KeizerHarm Aug 27 at 13:43
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    $\begingroup$ Chlorine36 and calcium41 have half-lives of around 100k years, and are quite soluble. Have to work out the doses (they are both beta emitters, so not particularly dangerous in small doses), but as far as lethality goes, just keeping the sea hot enough to stop passage without refrigerated breathing gear is enough. When you lungs are the coolest places around in a humid environment you drown pretty quickly. $\endgroup$ – RobbieB Aug 27 at 14:47
  • $\begingroup$ PuN03 and PuCl are likely to be soluble, at least soluble enough to make the sea pretty nasty (and warm), after all, you don't need much $\endgroup$ – James K Aug 27 at 14:54
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    $\begingroup$ +1, great idea. Please excuse my pedantry, but, what's nonphysical about radiation? $\endgroup$ – Emilio M Bumachar Aug 27 at 17:20
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    $\begingroup$ @JamesK, Plutonium is a lousy choice for this: it's an alpha emitter. A boat hull provides more than adequate shielding against alpha radiation. If you're trying to make a deadly radioactive sea, you really want something that gives off beta or neutron radiation, preferably with copious gamma radiation as well. $\endgroup$ – Mark Aug 27 at 21:43
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Two ideas! on a theme:

Lava (or at least molten rock)

Due to some wierd quirks of geology, the "seas" are molten, slow flowing rock. They don't have to even be completely liquid at all times, just enough to be so hot that any structures you build burst into flames. The gases given off by this should make traversing it very difficult. Conversely, it'll speed the invention of flight by giving you some excellent thermals - launch off the mountain with your glider, and you've got a good chance of making it over

I'd suggest that rather than actual geology, the aliens who set this up might have created, under the channels, some massive reactors - think vast amounts of uranium, or some elements with an appropriately long half life, to keep this rock constantly hot.

Steam

Similar to the above, you have a mass of radioactive elements/ area of high geological activity, and a flow of water down the sides of the mountains, from the rain.

This creates a strange, haunted looking cloud filled valley between the mountains, which creatures go into, but never emerge. It gets progressively hotter as you go down, and then the radiation kicks in, until creatures are either cooked or irradiated. It would look like a sea from the mountain tops, and could keep running for thousands of years. A thick layer of rock on top of the radioactive stuff should keep radiation leaks to a minimum.

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  • $\begingroup$ Steam is lighter than air, so if the valleys were filled with steam, wouldn't it quickly just rise out? $\endgroup$ – Tanner Swett Aug 27 at 20:15
  • $\begingroup$ @TannerSwett Set the heat levels so that the steam would condense well before hitting the top of the 5km mountains? $\endgroup$ – Kevin Arlin Aug 28 at 0:45
  • $\begingroup$ @Kevin Arlin that was my thought too, have the mountains act as a condenser... $\endgroup$ – lupe Aug 28 at 8:25
  • $\begingroup$ Then the mountains heat up until they're hot enough to keep the steam as steam and then it rises out. $\endgroup$ – user253751 Aug 28 at 17:08
  • $\begingroup$ It's an interesting physics question. I'd guess that roughly 100 degree differences could be easily dispersed by mountains - after all, they handle a lot more under the earth's crust. You'd get some interesting ecology on the non dead side of the mountains though -expect hot spring style lifeforms, tropical reefs, that sort of thing on the bits underwater, and serious cloud forest on the bits above water. It'd look fascinating - a mountain range, always shrouded in cloud, with lush rainforest. As people climb over, they vanish, never to be seen again. Definitely a religion or two based on this $\endgroup$ – lupe Sep 1 at 13:38
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Bacteria-infested water.

Yes, it should be toxic to every known form of life. Your civilization probably hasn't developed biology enough to know what bacteria are, so they are not known forms of life.

Long ago, the water was flowing uninterrupted, life was plenty and free. Then the ridges were raised, which also raised the sea floor in between, with no concern over anything living in between. All fish and other sea life died in droves and bacteria ate their remains, but all the corpses lead to an environment more acidic than the oceans. After a few hundred years it turned into a sludge of bacterial life adapted to the unusually acidic water, hunting for any bit of nutrients they can get, constantly evolving to become a stronger bacteria-on-bacteria predator. It's been that way ever since. Evaporation takes water away, but it quickly gets refilled by rain - and every rain brings an influx of new nutrients, during which the bacteria replenish the numbers lost during dry periods. Any bacteria that fall into the ocean outside die, because they are too adapted to the dead water.

It might look like normal, albeit stinking, sea water, but once the bacteria get into your body, they start to eat all the nutrients they can get their flagella on, whether it's your flesh or your organs, maybe even your bones. They don't survive in your body for long, because your body simply is not acidic enough, but they live long enough to be lethal if ingested in high enough doses.

All the mutual bacteriocide leads to buildups of methane, which frequently bubbles to the surface and causes small boats to sink. Methane bubbles are already a known phenomenon in real life, though usually not as a result of bacteria, because they don't have the numbers in normal oceans.

Essentially, the dead water would be a huge, bubbly swamp hole full of dead things.

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    $\begingroup$ As a bonus, your bacteria could regularly release huge amount of fermented gas into the water. Those large bubbles would randomly sink boats. youtu.be/C7A645F-3sk?t=13 $\endgroup$ – Eric Duminil Aug 28 at 9:06
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Mercury

It is quite toxic. It will be liquid, and stable as long as the mountains don't interact chemically with it. Since they were put there by intelligent beings with unlimited budget, that might be the case.

Any water raining on it will stay on top, because mercury does not dissolve in water and is way much denser. Since the mountains are artificial or divine, they can have mechanisms to drain any water.

XKCD What If #50 has this to say about a hypotethic ocean made of mercury:

Rowing a boat on a sea of mercury just might be possible.

Mercury is so dense that steel ball bearings float on the surface. Your boat would be so buoyant that you'd barely make a dent in the mercury, and you'd have to lean your weight into the paddle to get the end of it below the surface.

In the end, it certainly wouldn't be easy, and you wouldn't be able to move fast. But you could probably row a little bit.

You should probably avoid splash fights.

Some people thinking they are geniuses might try and project boats (or armor, since you can walk on liquid mercury) that would allow them to cross the mercury seas. But when that ocean is big enough to have waves, that will be quite troublesome. The waves would keep even jeeps from being able to cross.

Mercury will also eventually corrode iron, stainless steel and other materials. Aluminium will resist as long as it has an aluminium oxide layer protecting it, but damage the piece and you will slowly summon a demon.

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    $\begingroup$ I don't quite follow why mercury waves are so troublesome. Most materials are extremely buoyant in mercury, so it will be nigh impossible to sink a boat or drown in a mercury sea. I suppose you could get crushed to death by a falling wave more easily, but mercury's additional density will result in waves of rather low height compared to a watery ocean. Sailing over a mercury ocean seems like it would be easier than on water due to the massive boost in buoyancy. $\endgroup$ – Nuclear Hoagie Aug 26 at 21:26
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    $\begingroup$ I second @NuclearWang. Mercury is so heavy that I have trouble understanding where those mythical waves would come from. Something like a sailing iceboat would be well suited to traverse such a sea. $\endgroup$ – AlexP Aug 26 at 21:32
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    $\begingroup$ I'm not convinced that you get any waves at all, because a) mercury has a density 13.5 times that of water - you would require ridiculus wind-speeds to get waves worth to mention and b) the rivers/seas are in deep valleys which essentially allows only for wind blowing in 2 distinc directions to affect the "water". Then another point: Pure mercury is not toxic enough to fulfill the OP's requirements - you can literally drink mercury without incurring ANY health problems (that is assuming you do not have any internal wounds). $\endgroup$ – Anonymous Anonymous Aug 26 at 21:37
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    $\begingroup$ I think you could have serious issues with mercury vapor slowly toxifying the entire planet. It does evaporate, and this is a LOT of heavy metal poisoning due to the sheer volume of mercury around the world. It may even deplete the supply eventually just from evaporation. epa.gov/mercury/basic-information-about-mercury $\endgroup$ – DWKraus Aug 27 at 3:19
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    $\begingroup$ Mercury vapor will take forever to kill you. It also is going nowhere with 4km cliffs around it. As far as traversing goes, wooden boat, paint and sails, or wooden kiteboard. Anyway, after 20,000 years of erosion there's likely to be enough of a dirt layer on top to walk across. :) $\endgroup$ – RobbieB Aug 27 at 16:29
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Consider, instead of dead water, using dead air, namely co2. Regular Limnic eruptions combined with the mountains to contain the gasses would make it impossible to cross the barriers without a vehicle capable of carrying its own air.

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  • $\begingroup$ This is a duplicate of Ash's answer. $\endgroup$ – cowlinator Sep 1 at 18:57
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Tungsten hexafluoride gas on top of oil on top of the water

Tungsten hexafluoride gas is 11 times as heavy as air, toxic and relatively stable, so you don't have to worry about it blowing over the mountains, decaying, or reacting with oil.

Only three problems you might run into.

It will eventually react to rain, but the rain drops will drop straight through the oil, so they won't be in contact for very long, and the reaction isn't very powerful. Over tens of thousands of years, this might be an issue though.

Its boiling point is 68 degrees, so it will be a liquid if it drops below that temperature. You may be able to get away with this by changing the molecule somewhat, perhaps substituting the fluorides or something? could make a related molecule. If you find one that has not been researched ever, depends how realistic you want to make it, could just make up some properties based on what should be expected based on how other substitutions affect it.

Or, Sulfur hexafluoride. This is completely non-toxic but also completely inert. It being nontoxic doesn't change that you would still need enough oxygen for the entire journey, which hopefully is prohibitive enough in your world.

Hopefully this is a good starting point if not a solution!

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As some answers suggested, I would go around the "strictly" dead water concept and substitute it with:

  • strong currents / vortices that make navigation almost impossible, or a set of waterfalls...
  • extreme weather conditions (icebergs, for example by the time a civilisation evolves ice-breaker ships, flight cannot be much far behind)
  • a cultural / religious fear of crossing the "dead water". Maybe it is an acid as some suggested, people observed animals or fellow humans die in it by exhaling or attempting to dive and are now afraid to cross it. It can be possible in principle to navigate it but people just regard it as "dead water" and nobody is planning to bring a glass-canoe on the other side of the mountain just to explore. Again, by the time a civilization is scientifically evolved enough to attempt crossing, it probably is also very close to invent airplanes.
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    $\begingroup$ So - "Truman show" them in, probably a decent point that fear is more powerful than a surmountable obstacle anyway. I bet the guy lugging the glass canoe over the mountain looks like a real idiot when he finds it's just normal water all along. :p $\endgroup$ – Ash Aug 27 at 14:36
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Tall Butte Cliffs

Kind of a frame challenge, but if your mountains are instead rings with sheer cliffs and flat (or rugged) tops with water on both sides, nothing special aside from the durability of cliffs is really required. In order for one civilization to contact another they'd need to take a boat to their cliff, scale the wall, bring the boat up, transport it across the top of the cliff, lower it down the other side, sail to another civilization's cliff and repeat the whole tedious process.

This would restrict contact fairly harshly and pretty much eliminate the possibility of trade. Few, if any, goods would be worth the tremendous amount of effort required to do the above twice.

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    $\begingroup$ If it was high enough, the air on top could be unbreathable without special equipment! I like this answer $\endgroup$ – lupe Aug 27 at 15:21
  • $\begingroup$ Don't underestimate the power of people with spare time and suction cups! man climbs tower with suction cups. And if you are able to reach the top, you can set up a pulley system to bring other people and boats up there easily in the future. $\endgroup$ – Alexandre Aubrey Aug 27 at 19:33
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    $\begingroup$ Unless the cliff-faces are exceedingly smooth suction cups wouldn't work. It's not an impossible task, however it's expensive enough to be prohibitive, especially with no guarantee of return of investment. If each island is far enough away the others won't be visible from the top, making justifying further expense unlikely. And even if you do make it to another civ, whose to say they won't either kill you for being an invader or be so much nicer you never want to return. $\endgroup$ – aslum Aug 27 at 19:38
  • $\begingroup$ Creative, but remember that tall sheer cliffs would be vulnerable to erosion and unlikely to last long on geological time scales... Just something to consider! $\endgroup$ – Alex Aug 29 at 4:46
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Fish that drill through boat hulls, sink them, and burrow through human flesh, turning an entire boat of sailors into a sunken ship and skeletons in a matter of minutes.

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  • $\begingroup$ Or underwater drones from a long dead civilization that destroy anything that enters the waters. $\endgroup$ – David Aug 28 at 1:10
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Another, more chemistry based answer - if the sea had quantities of arsenic compounds, you could postulate a photo/chemosynthetic arsenate reducing bacterium living in the water. Arsine is incredibly toxic, heavy enough to stay in place without escaping over the mountains with appropriate wind conditions, and stinks to high heaven. Obviously there are lots of other candidates for nasty lethal gases evolved by bugs as well - hydrogen sulfide or phosphine would also work.

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Tidal flows of ordinary seawater

Tides on Earth follow the Moon around its orbit. We see rise and fall because of this but we also see very fast flow through narrow channels.

If the geometry of your canals was regular and formed by diamond-shaped mountain ranges around the "waist" of the Earth, there would be a constant high-velocity flow of water travelling along the direction of the equator. The water would always be trying to keep up with the moon and if the depth and width of the channels was just right the speed of this flow could be enormous* Add a few strategically placed seabed features and the whole thing could be a mass of deadly whirlpools.


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    $\begingroup$ mostly related to the speed the earth spins (the moon orbits about 4% slower than that) $\endgroup$ – Jasen Aug 28 at 10:39
  • $\begingroup$ @Jasen - Thanks - If you know a lot about this, I'd be interested in your help. P.S. do you have a link to this information? $\endgroup$ – chasly - supports Monica Aug 28 at 11:06
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    $\begingroup$ for a link "wikipedia tides" the moon orbits every approx 28 days (sidereal or solar frame of reference) . but an an observer on the ground sees it rise and set every 1-1/28 days (about 24h 50m I guess) and during that period sees two high and two low tides,. $\endgroup$ – Jasen Aug 28 at 11:22
  • $\begingroup$ @Jasen - Thanks for your help. As an answer has been accepted and mine has not received many votes, I won't continue researching my suggestion. I appreciate your info. $\endgroup$ – chasly - supports Monica Aug 29 at 14:57
  • $\begingroup$ That's not how tides work, though. You only get fast flow into areas that are connected to the main body of water by narrow passages. If the barrier seas are contiguous and wide, tidal flows will probably be minimal. You don't notice tides in the oceans. $\endgroup$ – RobbieB Sep 1 at 14:16
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Given your special geography, you can do this with regular seawater.

Each of your barricaded-off regions is essentially a tectonic plate. The mountains were formed by the plates smashing into each other. The plates have since drifted apart, leaving oceanic rifts between them. These rifts are deep enough that they allow hot gases from the planet's mantle to escape, causing a large volume of bubbles that continuously aerate the seas above the rifts. This aerated water is significantly less dense than regular seawater. As a result, it has very little buoyancy and any boat, swimmer, or normal aquatic animal will sink in it. If you like, you could design completely new species of fauna that have special adaptations for this environment but would seem completely alien to the outside world.

By using regular seawater, you avoid many of the problems mentioned in your question. Rain falling into the "dead seas" won't impact their deadliness. Any water that evaporates out of them and rains down elsewhere will not carry its deadly properties with it. The water in the "dead" seas is no more or less stable than the water in the "normal" seas.

The only remaining issue is toxicity. If you want consumption of this water to be deadly, the easiest solution would be to have one of the materials expelled from the rifts be a naturally-occurring toxic substance that dissolves in water (such as arsenic, which is a known contaminant of water supplies and difficult to get rid of).

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Methane hydrate

Of course this solid would make up only the bottom of the sea. The sea itself could be pretty normal water (or something else). It is speculated that some of the disappearances in the bermuda triangle could be due to release of large methane bubbles that annihilate the buyoancy of a ship that happens to cross above the surfacing bubble.

So by having massive deposits of methane on the sea floor with more or less constant bubble formation all over you could effectively prevent any ship from crossing.

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  • $\begingroup$ That won't be stable for the required time frame because of the constant venting all the hydrate will fairly quickly turn into gas and the gasses escape into the atmosphere. $\endgroup$ – Tonny Aug 27 at 10:34
  • $\begingroup$ Well if you handwave those structures in the first place you can make the deposits humongous too. I dare say it does not need to be a constant gas stream but sufficient bubbles every now and then to make a successful passage very unlikely. $\endgroup$ – And Aug 27 at 11:19
  • $\begingroup$ Depositing huge quantities of methane into the atmosphere would probably heat up the planet, thus interfering with the experiment. $\endgroup$ – KeizerHarm Aug 27 at 11:55
  • $\begingroup$ @KeizerHarm Not to mention the smell... $\endgroup$ – Tonny Aug 27 at 12:18
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    $\begingroup$ Tonny, methane is odourless. The clathrate is stable under pressure, so you also need to postulate a mechanism to disturb these deposits frequently enough to bubble but not to exhaust them. This is not trivial - there is an immense amount of methane clathrate in our oceans that basically just sits there minding its own business, and we almost never see methane burps in the sea. $\endgroup$ – RobbieB Aug 27 at 16:14
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Lets look at all the features this sea would need to have to fulfill your goal.

  1. It must be quickly lethal. If it's too slow, transit will not be impossible, it'll just be relegated to desperate or condemned people. It's not an impassable barrier. Mercury, organomercury compounds, and many other poisons fail this requirement.
  2. It must be extremely corrosive. If it's not, there's nothing preventing people from building boats or pontoon bridges. The sea must actively destroy any constructs put in it, or once again, it's only difficult to cross not impossible. But the sea floor must not be corroded or else the death liquid will either eventually leak out into the wider world or it will get used up corroding rock instead of foolish explorers.
  3. It must not survive evaporation. If it does, it won't stay confined to the death seas, but rather will be spread by the wind. This combined with #2 rule out normal bases and acids.

I don't think there's any real liquid that can solve all these problems. It's going to have to be magic or handwavium.

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    $\begingroup$ It doesn't have to be amazingly quickly - just quick enough to kill people off consistently before they can climb down, cross the sea, and climb back up again. $\endgroup$ – Ben Barden Aug 27 at 13:18
  • $\begingroup$ There's stuff that fits, but nothing that will be at the same temperature as water -we're going to heavily stretch the definition of sea to make this work $\endgroup$ – lupe Aug 27 at 13:33
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I'm astounded no one has just suggested brine. You really don't have to do anything at all to make these 'dead seas' dead - it's in the name, just look to the real life Dead Sea. There's no outlet from the dead sea here, so any rainfall will slowly dissolve the mountains surrounding the sea and accumulate in the water. After a while the water is going to be absolutely nasty. There will be some bacteria that will likely survive, but not much else. Drinking the water is out of the question - near instant death. If it gets in your eyes, it will blind you. Mixed with the surrounding mountains making getting a boat into the water next to impossible, and you've got a very inhospitable environment. For an added bonus you can make the sea level within the mountains much lower than outside them, and get very hot temperatures. Dehydration will come quickly to anyone trying to pass, and there won't be any drinkable water.

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  • $\begingroup$ What makes this sea difficult to traverse? I don't see much difference from a normal ocean, which is also undrinkable. Just bring a lightweight boat and plenty of water. $\endgroup$ – Nuclear Hoagie Aug 28 at 15:38
  • $\begingroup$ @Nuclear Wang, that's like saying just build a light rocket with plenty of fuel, though I agree, the challenge of traversing a sea of brine is long gone. The sea would be toxic, but nothing we can't handle, and nothing we can't cross... so the narrative would seem a hard sell... $\endgroup$ – Alex Aug 29 at 4:50
  • $\begingroup$ Actually, if the "live" seas are all fresh, this is something completely new to the people. Further, people coming down the mountain would believe the water potable (drinkable), and drink. $\endgroup$ – David G. Aug 30 at 0:44
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I'm surprised no one has suggested Boiling Water yet.

All this requires is a heat source under the water -- geothermal energy would work a treat. Nuclear power sources (i.e., radioactive isotopes at the bottom) could also be used.

Water is of course very stable and rain replenishing it is a good thing. You could also have the water be saturated with salts, which would be stable and also raise the boiling point above 100'C / 212'F.

The heat would be deadly to humans. The air immediately above the water would be very hot -- you should have large amounts of steam bubbling up.

The only way to cross would be either flying well above the water or having some kind of cooling system. Primitive cooling systems (e.g. insulation plus ice) would not last long enough to cross several kilometers of boiling water. Engineering a cooling system that could last for many hours would probably be more difficult than attaining flight.


On a side note: Heavier-than-air flight was first achieved in 1903, but lighter-than-air flight happened 121 years earlier. The English channel was crossed by air in 1785. So when it comes to the technology needed to cross, you should think around 1800, not 1900.

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Impassable jungle

The water would not be dead, but too much alive. Till 19th century, there was practically impossible for western civilization to build anything in tropical jungle and exploration was only barely possible. Diseases, oppressive hot humid air, dangerous insects, leeches and other nasty fauna and flora, dense vegetation that quickly grows back. Till today there are impenetrable places like Darién gap. You might inspire yourself by looking up troublesome construction of Panama canal, and combine jungle and mangrove forests so that there isn't even any soil to build anything on.

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Quicksand

Quicksand is a shear thinning non-Newtonian fluid: when undisturbed, it often appears to be solid ("gel" form), but a less than 1% change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form). After an initial disturbance—such as a person attempting to walk on it—the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to decrease suddenly. Someone stepping on it will start to sink. To move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of 0.01 m/s would require the same amount of force as needed to lift a car. https://en.wikipedia.org/wiki/Quicksand

Good luck trying to row a boat through quicksand!

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    $\begingroup$ Is it stable over 40k years? Or can you set up (subterranean) geographical features so that it will remain in place over that time period? $\endgroup$ – KeizerHarm Aug 27 at 12:09
  • $\begingroup$ @KeizerHarm - Can I suggest you put things like width of canals and length of time required into the question itself. Otherwise we are guessing. Thanks. $\endgroup$ – chasly - supports Monica Aug 27 at 12:14
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    $\begingroup$ I did refer to the tens of thousands of years in the main question, several times. Width of the canal feels a bit less important. $\endgroup$ – KeizerHarm Aug 27 at 12:22
  • $\begingroup$ The channels have to be wide, at least a kilometer, or a bridge is going to be relatively easy to work out...or, like, a causeway, collapse half the mountain into it with dynamite, and you're a good distance in.. it's why it needs to destroy material added to it. $\endgroup$ – lupe Aug 27 at 13:37
  • $\begingroup$ You might not be able to row a boat through quicksand, but you can build a pontoon-like bridge across it just fine. $\endgroup$ – Mark Aug 27 at 22:00
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Something different:

Tsunamis. The mountain ridges are caused by strong geographical disturbances which cause relatively shallow depth earthquakes. These quakes trigger deadly waves that ravage in the channel from side to side causing a very dangerous ripple to cross.

To address the toxicity factor; vulcanoes and rifts in the seabed are common occurance in this nightmare sea.

The source of the anomaly could be the creators' machines working to correct the height of the barriers.

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    $\begingroup$ Out in the open ocean, the wave of a tsunami is only a few feet high. The reason they're so dangerous on the shore is that, unlike ordinary waves, the disturbance of a tsunami goes all the way down to the seabed, and this disturbance stacks up on itself as the water gets shallower. $\endgroup$ – Mark Aug 27 at 21:53
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Personally I'd avoid the complexity of chemical soups and go with hypersalinated water, like that in the Dead Sea. Ingesting it would be toxic, it would evaporate into water, and the mountains on either side would create a rain shadow preventing excessive dilution.

It would be harsh on the skin but not necessarily fatal. The only part that might not work with your purpose is that it would be much more buoyant than ocean water. If it were my world I'd infest it with some critter that would prevent trying to cross it by boat.

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Probably the most likely thing based on the real abundance of things in the Earth's crust would be a concentrated ammonia solution. Like water, ammonia is a simple molecule that, unlike a lot of others here, is actually very abundant in the solar system (and other systems, generally).

On Earth it is destroyed by atmospheric and biological processes, but you could handwave a semi-engineered geo-chemical mechanism for continually replenishing the ammonia lost due to environmental effects, etc. This is effectively a large infrastructure project, and the usual goal with these is to use the most abundant and cheap material available that satisfies the requirements.

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