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I had an idea for a sci-fantasy story set in a far future where there are primitive human settlements at the base of Mauna Kea who are unaware that they're actually dwelling on the sea floor of the Pacific Ocean. The ocean surrounding the islands of Hawaii has been drained and there's an immense circular wall built using "advanced" technology to keep the ocean at bay.

What effect would stuff like atmospheric pressure have on people on the sea floor in this scenario? Would temperatures on the sea floor be absurdly high? What would the terrain look like after thousands of years?

What sort of flora, if any, would grow below sea level or on the drained land?

This is assuming that the Earth’s oceans have not all drained away, just within this cofferdam like structure surrounding Mauna Kea.

If the sea floor would not be suitable for settlement, what broader scenarios or technological work arounds would need to exist to make it possible?

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    $\begingroup$ Mauna Kea lies on Hawai'i, the largest island of the eponymous archipelago. It's the tallest peak on the island but not the most massive mountain. There is also no water immediately surrounding the mountain. So you might want to pick another seamount for the aesthetic you have in mind. $\endgroup$
    – KeizerHarm
    Jun 18, 2021 at 22:45
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    $\begingroup$ I'm envisioning a wall hundreds of miles away from Hawaii so that the base of Mauna Kea would be dry land. The base of Mauna Loa would also be dry land. I'm wondering how feasible human habitation would be at the drained sea floor at the foot of these mountains. $\endgroup$
    – Gwood
    Jun 18, 2021 at 22:56
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    $\begingroup$ I edited the post to say "drain the water around Hawaii", to make the size of the drained area clear. $\endgroup$
    – Gwood
    Jun 18, 2021 at 23:09
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    $\begingroup$ Do you have continuous pumping of water out of the base of your coffer dam? Because Hawaii are wet, and all those rivers and streams will work continuously to refill the area. $\endgroup$
    – Alexander
    Jun 19, 2021 at 3:34
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    $\begingroup$ I've only one item to add, so I won't make it a full answer: you're going to have some serious pump problems. Hawaii holds two positions on the top-10 "wettest locations on earth by annual rainfall" list. Yes, it's the proverbial drop-in-the-bucket by ocean standards - but you shouldn't underestimate how much water you'll need to remove day after day after day.... $\endgroup$
    – JBH
    Jun 19, 2021 at 7:06

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Time to make some assumptions to make the primary question easier to answer.

  • Putting the radius of the wall at 100Km. The big island is very approximately 50Km radius. This will put the wall far enough away to not have the ~6Km high wall not so obvious and actually have some room to live/farm.
  • Sump pumps were installed at the same time to prevent infill by water.
  • Wall thickness is 1m, wall height extends 10m above mean sea level. This allows only the most extreme storm water over. Allows weather systems to pass over.
  • Coffer dam extends 500-1000m into ocean floor this is to prevent water flowing under the damn.
  • Coffer dam is constructed of unobtainium for its structural strength.
  • After dam placement the enclosed ocean water is pumped out over the next year.

Things to expect to happen after placement, first year

  • Sea life that was living close to the surface dies.
  • Many land slides as the angle of repose is much steeper below water then above.
  • Due to pressure changes food availability changes, almost 90% of the life that was in the waters surrounding the island are killed.
  • Many significant(7-8 mostly probably a few 9+), massive earthquakes due to changing mass distribution.
  • Many rocks/minerals that were stable under ocean, no longer are. E.g. all gas hydrates would melt/sublimate

Over the next hundred years

  • Slow continued desalinization of the prior sea bed.
  • Weather patterns change initially to be much drier, mass loss of biomass on the island. less water to moderate temperatures. This changes as the ocean floor warms up.
  • Mass starvation of any people living there, less fish, less water for farming.
  • Weird chemistry in the surrounding valley due to higher temperatures and large pressure variances.
  • Some plants start colonizing down slope.
  • Continued massive earthquakes as the area lifted up.
  • Magma eruptions would probably start occurring around the south base of the mountain due to changing path of least resistance to release magma pressure.
  • Temperature of the ocean flow starts rising, without any cooling over a 100K rise would be expected.
  • The increased temperature starts driving storms. which start providing significant cooling.

100 to one million years

mostly skipping this.

  • weather formations stabilize.

Over the next 1-10 million years (assuming wall and pumps maintained)

  • Earth quakes back to pre dam levels
  • the center will have risen enough to have gentle slope down from the center to the wall.
  • New volcanic cone well formed.
  • Plants extend range down much of the slope. By this point plants would have reached the prior ocean floor, higher cooler spots would also be colonized.
  • The valley bottom slowly greens, but high temperatures and or pressures keep most greenery and certainly farming away.
  • Humans follow plant growth further down slope. But are limited due to the temperature and storms.
  • The massive storm system cause the storm gods to supplant the volcano gods as being the most powerful.
  • Air temperature in the bottom to start rising. This will start driving some very large storms. In theory 30K/Km or 180K above surface so without cooling temperatures will be tending to well above boiling across the valley
  • Air pressure will depend on air temperature, If it is 30K/km then air pressure will be about the same as surface. If it is 0K/km air pressure will could be as much as double normal air pressure. Between temperature and pressure there is going to be some fantastically powerful weather systems happening.

last two points are derived from 'Pressure and density of air in mines'.

Possible weather trend

Not certain but it seems that a massive doughnut shaped near perpetual storm would form. Colder moisture laden air would down draft all around the wall into the valley bottom where it would heat up, picking up more water flowing off the center then start rising. Dropping moisture as it cooled over the center. Then fall back down towards the wall. Depending on how much cloud this could really limit food crops.

Closing

The valley floor/prior sea bed will most likely be too hot to live. It needs constant cooling to keep temperature down. The constant rain and river flow would keep it from going into the boiling range. Much if not all of valley could be well above comfortable living. If it is cool enough to live then the higher air pressure will be a complication. How much complication I am not sure.

How could the valley bottom be more viable as living space? In addition to the ocean pumps a large network of heat pumps to keep the heat below 30-40C would probably do it. Perhaps. This would also calm the storms.

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    $\begingroup$ 1-10 million years for vegetation to take over seems inconsistent with how quickly forests have taken over since the ice age just 20,000 years ago. Is the difference due to the salt? I don't think air data for mines enclosed in rock is applicable; otherwise valleys would be furnaces. 6.5K/km seems to be the rate for above-ground temperatures, from atmosphere insulation instead of geological heat, for max 60°C temperature. Otherwise, I quite liked this answer and found it well researched. The donut storm seems like a good way to solve the heat problem either way IMO. $\endgroup$
    – Will Chen
    Jun 19, 2021 at 3:10
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    $\begingroup$ Thank you so much for the thought you put into this - you've given me plenty of ideas and these details will help me add a healthy dose of realism into the story. Interpreting your breakdown through my scientific illiteracy, I'm envisioning hardscrabble human settlements on the slope of the new volcanic cone, overlooking the stormy ex-seabed/valley below? Stormy, cloudy (but with occasional sun on the slope of the volcano?), windswept . . . would it also seem reasonable to assume that over millions of years humanity and other fauna would have evolved physically and culturally to adapt? $\endgroup$
    – Gwood
    Jun 19, 2021 at 13:20
  • $\begingroup$ I think you underestimated the number of land slides and erosion that would happen. Right now, the main island of Hawaii is being split with a huge mass getting ready to slide. On the north side of Oahu, there have been so many slides driven by heavy rain fall that there is large cliff. The oldest island is less than 5 million years old. The rest of the islands have already eroded to below sea level. Few volcanoes last very long. Most erode quite quickly as the lava crumbles. $\endgroup$
    – David R
    Jun 19, 2021 at 14:31
  • $\begingroup$ Don't forget the new volcano that is currently forming, Lōʻihi Seamount. It is expected to reach sea level in the next 10,000 years. $\endgroup$
    – David R
    Jun 19, 2021 at 14:33
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    $\begingroup$ Besides that... The sudden removal of 193584960660000 tons (circle 100km radius, 6km deep, mostly seawater) of pressure in a very restricted area will be like applying a vacuum pump to a pimple. What is the chance of popping one or all of the three big magma reservoirs under Hawaii? Or possibly even causing a bleeder by drawing the primary Hawaii Mantle Plume to the surface? Hello Siberian Traps Mark II, the Tropical Edition! $\endgroup$
    – PcMan
    Jun 20, 2021 at 16:32
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Disclaimer: The science and methodology here are going to be a little sketchy. Hopefully that's fine. IMO this kind of speculative worldbuilding really only needs to give a ballpark number figure; the important thing is the consequences of approximately where it ends up.

Skip to the end for the most exciting conclusion.

Summary:

  • You might get oxygen poisoning.

  • You might get carbon dioxide asphyxiation and volcanic gas poisoning.

  • You might get giant animals, evolved to both exploit and defend themselves against the oxygen.

  • Ecology will probably be limited by nutrient availability, following a normal ecosystem development pattern.

  • Be careful how you remove the oceans, so you don't leave a thick layer of dried sea salt that kills everything.

  • You might get surface temperatures up to 60°C, but maybe not, and you can improve the odds if you shape the walls to create winds and conduct heat.

  • The whole place will flood with nowhere for rainfall to drain to. Fixing this probably means either lots of pumps, or a (likely arid) climate with evaporation and precipitation in equilibrium.

  • Earthquakes will probably destroy your wall as the thin sea floor crust rises upwards to make up for the lost weight of the water above it.

  • Every global coastal city outside of the walls will permanently flood due to the volume of water removed for your cofferdam.

  • Hydrothermal vents will be interesting to see outside of the ocean.

  • The conditions at the bottom are perfect for biologically realistic giant fire-breathing dragons.


The sea around Hawaii is apparently up to 6km deep in some places— Around the same difference in elevation as the surface and the peak of Mount Everest, where the air is less than half as dense. Higher air pressure with the same elements also means higher partial pressure of oxygen. This could be immediately dangerous to unadapted humans:

  • Oxygen Poisoning — Too much oxygen or oxygen at too high a pressure can cause disorientation and damage to a lot of organs.

Hawaii also may not be the best place for this. Carbon dioxide is aleady heavier than air, and every now and then a cloud of it will build up in a valley and smother everyone there. Volcanoes emit a lot of gas, lots of it more toxic than CO2.

Even if the gasses enough aren't enough displace all the oxygen and outright smother everything, the higher pressure might be enough to force enough of it through the lungs to turn blood dangerously acidic— Not to mention that chronic exposure to more dangerous volcanic gasses would probably be unhealthy. With the wall hundreds of miles away you'd probably be fine, but you might have problems if it were much smaller or during a particularly volcanically active time. Maybe there will be pockets of immediately lethal gas near the lowesst parts of the sea floor, which your people will turn into myths about the underworld.

Depending on how bad all this is, species could experience some pretty harsh and aggressive/fast selection pressures to adapt.

  • Gigantism — Growing bigger bodies is both an opportunity afforded by higher pressure and a way of coping with greater oxygen.

A lot of the biggest prehistorical organisms that would not be biologically viable today existed during periods of higher atmospheric oxygen content. Griffinflies and pterosaurs peaked during the Carboniferous and Cretaceous, when the atmosphere held as much as 50% more oxygen than it does today. Having more oxygen in the air should allow the body to consume more power without that much more expensive of a respiratory system, meaning they can grow much, much larger if there's a competitive advantage to doing so. At the same time, it may also create an advantage to doing so— If the levels of oxygen and other gasses are enough to damage the tissues and organs of a normal-sized creature, then growing a bigger body could dilute those gasses back down to safely usable levels.

  • Biodiversity — Are you leaving nutrients in place?

As long as there's sunlight, I don't see any definite reason why there shouldn't be fairly normal flora. Evolution doesn't usually just create life from thin air though, so I think the flora that ends up growing down there will largely be restricted to the flora that's already in the region to be able to spread, and its descendants. At first you'll probably be limited by nutrient availability— There's apparently a lot of fish and stuff in the seas around Hawaii, so if you're willing to let that die and drop to the dry sea floor, you could get a head start. Otherwise, you might have to wait for the usual pioneer species like lichen, algae, and simple plants to move in and break down the rocks into soil.

  • Salt Flats — If you're drying out the oceans by evaporating the water though— As opposed to pumping or otherwise moving it out— Then you'll get a thick layer of salt on the surface. In this case, obviously basically nothing will grow except for some extremophile species of Archaea and such.

On your timeline of just a couple thousand years, I don't think any new species will be able to evolve, although you might be able to get pygmy and giant versions of existing species— This could be a combination of the unique challenges of higher air pressure, and the usual gigantism and dwarfism that tends to happen in isolated environments.

  • Uninhabitable Temperatures — You might need to create a permanent hurricane just to avoid cooking alive in 60° heat.

Below 10km altitude, temperatures apparently increase by around 6.5°C for every kilometer downwards. If this trend continues below sea level, the lowest parts of the dry sea floor will be around 60°C (140°F).

To work around this, you could have very strong winds. The sharp walls of a cofferdam hold the promise of being able to create a permanent hurricane-style vortex— If the axis of this hurricane is vertical (I.E. it swirls around the center peak), then the winds could be used help animals cool off. If the axis of this hurricane is tangential to the surface (I.E. it swirls vertically up against the wall of the cofferdam), like in Howard Taylor's Eina-Afa spaceship, then it could serve to mix up air from hotter bottom and cooler upper atmosphere, keeping the temperature at the bottom reasonable. (Note that this would probably exacerbate the next problem of flooding, though.)

The very edges of the cofferdam will also be in shade a lot, which should reduce temperature immediately around them a a lot. But in other times of the day, they could potentially reflect extra sunlight onto the ground if they're a light colour.

Cloud cover would also provide shade, and a massive pressure and temperature gradient surrounded by ocean should have lots of chances to form it.

The ocean, as always, also holds a lot of promise as a heat sink. The best way to solve the potential temperature problem would probably be to ensure lots of heat transfer to and from the seas, by having strong winds and possibly building the cofferdam out of heat pipes lined with cooling fins or something.

Note that even if the temperatures do increase to 60°C in some places, Hawaii's already a diverse place with multiple biomes, and not all parts will have the same climate. The seas immediately around the islands are shallower, and should stay in the 30-40°C range even in this model.

If high temperatures do end up happening and remain unmitigated, they will obviously throw a spanner in the normal ecosystems that might otherwise be able to develop there. However, it's not quite the same as turning the entire region into a desert as it will probably have quite a bit of rainfall too.

  • Rainfall and Stability — Draining the ocean means nothing if it fills itself right back up again. This place might have to become a desert, or be permanently filled with steam, just to stay dry.

Normal dry land isn't dry because it's been drained once. Normal dry land is dry because it's higher than sea level, so rainfall on it will flow right back down to the ocean.

That can't happen with your cofferdam country, so there will need to be some other mechanism to keep it dry. Either there are pumps, geothermal boilers, or other specialized mechanisms to actively remove water, or the rates of evaporation and rainfall are naturally equivalent to those of a desert.

Some parts of Hawaii apparently already get quite a bit of rain. With the rapid changes in pressure and temperature caused by the cofferdam, this could potentially get worse­— I don't think there would be basically any rain at all over most of the surface of the sea floor, where the higher pressure and temperature allow the air to hold an insane amount of water without it condensing, but the walls of the cofferdam and the slope of the central island and peak will probably form clouds from that moist air.

The Netherlands famously have a lot of stuff built below sea level. They might be a good reference for how this kind of system could work.

  • Earthquakes — Removing the weight of the ocean from the thin oceanic crust seems dangerous.

Hawaii already gets thousands of earthquakes every year. The depth of the water you're removing is about the same as the thickness of the ocean crust beneath it, and rocks are only about 2-3 times as dense as water— In essence, you're basically stripping off a quarter of the weight of the entire surface of the Earth in that region, while doing nothing to reduces the forces of pressure and hydrostatic equilibrium that were holding up that weight before.

I would not be surprised to see the sea floor eventually rebound upwards. It would in theory have to rise by around two kilometers to make up for the lost weight of the water above it, so I'm going to say to expect up to a couple hundred meters of rise as a conservative estimate. And being made out of dense oceanic crust, it will likely do this in the violent, shaking sort of way that tends to bode poorly for large, rigid structures. Your cofferdam walls had better be built out of rubber, or something equally flexible.

  • Apocalyptic Worldwide Flooding — All that water has to go somewhere.

A 200-mile radius volume 5 km deep is around 1,600,000 cubic kilometers. That doesn't account for the volume of the islands, but nor does it account for the length of the island chain, and 200 miles is the minimum radius that fits the query's description of "hundreds of miles away", so this is probably altogether a conservative estimate of the amount of water involved.

Spread across the surface area of the Earth's oceans, this will increase sea levels by at least 5 meters. Many major coastal cities are going to be flooded.

  • HydroThermal Vents — There might be a completely unique biome. Or it might poison everything around it.

There's a couple of active hydrothermal vents around Hawaii. These are a source of heat, and they regularly pump out chemicals that can be both nutritious and poisonous. I don't really know what would happen if you dried them out, and I don't suppose that's something that anyone's probably looked into too much. Maybe they'll become inactive without a source of water, maybe they'll behave the same as a normal volcanic feature on land, maybe they'll form a new type of biome around it not found anywhere else on Earth, or maybe they'll spew out so much gas that they'll fill the entire basin with poison (but probably not).


  • Flight?With great oxygen comes great power, with great pressure comes great density, and with great power and great atmospheric density comes FLIGHT!

Pressure at the Earth's surface should be equal to the weight of the column of air above per area. Pressure below will be equal to that weight, plus the weight of any further air between sea level and the new surface.

However, the density of the air will also increase with each extra depth of pressure, which means the increase can't be modelled linearly.

Air weighs around 1.2kg/m³ at sea level, which translates to an instantaneous rate of increase in pressure per depth of 0.013kPa/m. That's (1.3/10000)/m relative to the sea level atmospheric pressure of around 100kPa, and they together map linearly to the increase of density (and thus rate of further pressure increase) further down, so I think that means I can model pressure below sea level as 100*1.00013**(depth/m) kPa. That model gets me to a ballpark within the accuracy of the numbers I based it on when I test it with the height of mount Everest, so I'm going to assume it's good enough.

(Hawaii's small enough that I'm treating the volume of the rest of the atmosphere as infinite. Temperature and humidity aren't accounted for.)

That means that the air at the lowest parts of the ocean floor will have a pressure from 190kPa to 220kPa, about twice as high as at sea level.

It will have an effective oxygen concentration of 42%, and a density 2X that at sea level, with both more oxygen available for life and more mass for wings to push off of.

35% O2 was enough for dragonflies to grow to the size of eagles. 30% was enough for pterosaurs to grow to the size of school busses, with armoured skulls and wings that could take them across continents. And that was without any significant increase in atmospheric density.

With 42% O2-equivalence, and air that's twice as thick? If you play your cards right, you're going to get dragons.

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  • $\begingroup$ I am pretty certain that one doesn't get oxygen poisoning at about 2 atm pressure, or about 0.4 atm partial pressure of oxygen. $\endgroup$
    – AlexP
    Jun 19, 2021 at 4:15
  • $\begingroup$ @AlexP Wikipedia says the lungs start having problems at 50kPa partial pressure, which is about 2.4 atm equivalent (0.5 atm partial pressure)... So yeah, close, but not quite. But I'm pretty sure that's acute. Chronic damage probably occurs at a lower threshold (including even at 1atm), at least with enough of a chance to be worth mentioning. $\endgroup$
    – Will Chen
    Jun 19, 2021 at 4:25
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    $\begingroup$ Dragons!?!? . . . bruh . . . thank you so much for your thoughtful and detailed comment - you've given me a ton of ideas. Deadly, alien environments and potential catastrophe . . . makes me think this world could be fertile ground for drama if not for agriculture ;) $\endgroup$
    – Gwood
    Jun 19, 2021 at 13:04
  • $\begingroup$ I try not to admit to myself very often that an answer is better than I would have done. Great ideas! $\endgroup$ Jun 19, 2021 at 16:27
  • $\begingroup$ It really is amazing, I'll be using Will's response as a reference for sure. All the responses so far have been better than I could have hoped for. $\endgroup$
    – Gwood
    Jun 19, 2021 at 16:34
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The geographical setting is a good kind of mysterious

This reminds of Larry Niven’s Ringworld series, in that you have a cornucopia of plot mechanisms from the texture, terrain, and (possibly) crumbling “old but advanced” technology. I can’t help but wonder who built that there? And why? Are they returning, or are these “primitive humans” actually a logical consequence of advanced humans who have imploded? Great idea.

The physics of a deep valley without water

A quick look at Google Oceans shows Mauna Kea stretches down 6000 metres or almost 4 miles below sea level. You said the water is removed so we’re only dealing with atmospheric pressure here. This results in an atmospheric pressure of roughly double that at sea level, or about 28 psi. At this pressure water boils at 120 C. In other words not a huge difference if your settlers are using fire to cook rather than some remaining ‘ancient tech’ ovens.

The ground is hot, but the floor isn’t lava

According to the [British Geological Survey][1] if you were to drill 5000 m below sea level then you would find the ground temperatures hovering around 130 C or 266 F. Now this is technically open to air and not drilled so it may be a lot cooler due to the wind, rain and other elements. The ocean floor, however, is known for steaming, acidic, geothermal vents. I’d assume these remained and are still active. Active that is, unless the ancient tech is covering them to harness energy or just as safety valves. Such geysers could be useful or a danger to the society.

The lay of the land

It would probably look quite silty, like the ocean floor. So effectively it would be a giant beach surrounded by walls. There would probably be significant volcanic cliffs although Mauna Kea is dormant so I wouldn’t expect new lava.

Flora and fauna

Oof these folks would have a hard life with vegetation and farming. Although the Hawai’i islands are quite lush; many of the seeds of these plants can’t drift that far over the ocean to the next island. Except coconuts. So I’d expect palm trees and a society revolving around palm oil, coconut flesh, etc. Kind of like Easter Island. Even if other seeds did make it there, it’s anybody’s guess if they could set root in an ocean floor type of silt. Perhaps some sea kelp or amphibious plants could have adapted fast enough. For sure you’d see bacterial slime, algae, and mould like you do everywhere.

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    $\begingroup$ If the temperature of the sea bed was 130 degrees, one would expect that the water laying on top of it would be above the boiling point. $\endgroup$ Jun 19, 2021 at 3:55
  • $\begingroup$ Thank you! The comparison to Easter Island makes it much easier for me to envision cultural analogs. Also the possibility for active geysers and how that contribute to folklore . . . I'll check out the Ringworld series. $\endgroup$
    – Gwood
    Jun 19, 2021 at 12:59
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The dam

Each island would get its own dam - it can best be placed on to of the Arch around the island, the dam will become a giant ring shaped structure extending 3500m to the bottom of the ocean and sticking out above the ocean, say 500-1000m, depending on safety regulations. See picture below.

Suppose mr Bezos and mr Musk are living on Hawaii and both are prepared to invest, total say 350B for the first part of the project (ca 30 years). At first it involves dumping gigatons (teratons?) of sand on the Arch, to reduce the ocean depth. The sand could be won on some nearby island groups, that will be evacuated as a result of climate change anyway. The sand from other islands will also be dumped on Hawaii itself, to prepare build the actual dam in concrete. After the first phase - Bezos long gone, there will be another billionaire sponsor - a metal mould is constructed (a cage) and remaining sand is used to fill the mould, so a concrete wall will be formed, with a pyramidal profile. On either side of the dam, at some point there will be ship elevator on either side, and a canal on top of the dam.

When the dam is finished, Hawaiians have gained 200% land.. you can live on top of the dam.. build harbours there.. Say the height of the dam needs to be 4 miles, ca 2 miles of that would be the concrete part.. the flattened pyramid profile of the concrete dam will be about 20-40 miles wide at the base, on top of the sand dropped on the Arch.. you need to wait ca 10 years before all that sand has stabilized. The structure would have a flat top of say, 5-10 miles, depending on calculations. As sea level is rising, the pressure of the surrounding ocean water will be enormous. A very rigid and strong ring dam is needed. People wouldn't be required to live down in the trough at all. The trough will remain water, at first.

As pointed out above, getting all water out of the trough would involve endless pumping.. and the result is not really a healthy environment. And also unsafe, because of the height difference and the pressure of the ocean outside the dam. The through needs to be filled up. This will be a project comparible to the dam itself, in size. Issue is.. how to get all that sand over the dam, inside the trough ? And the water should be moved also, in that scenario.. I regard this as an open end in my story, this submit is about the dam itself.

enter image description here

enter image description here

https://www.mbari.org/flexural-arch/

https://volcano.oregonstate.edu/hawaiian

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I am going to posit a dissenting opinion.

If the wall/dam is built 200 km. from the coastal line of the islands, then a 6 km. high wall is going to be inconsequential to the 200 km. of new flat land. Methinks the other posters are thinking it would be like a canyon, perhaps like the Grand Canyon, very high walls in relation to the width. It would be exactly the opposite - a wide shallow (comparatively to the wall height) saucer. One would still get weather patterns in this basin. 400 km, from wall to wall is a substantial distance for winds to pick up and flow over the 6 km. high wall (think a four hour drive at 100 km. per hour). There would NOT be a stagnant layer of atmosphere captured within the 'valley'. The wind patterns, after all, have the entire Pacific Ocean to build up a good headway. A six km. drop over 400 km. would be hardly noticeable.

And with so much air flow, their would be no expectation for the surface to be hot, any more than there is an expectation for the surface to be hot ANYWHERE on Earth. It would not be insulated by a 6 km, thick layer of earth, such that the heat could not escape, but it would be exposed to air currents like any other land mass. Winds and rain would be constantly flowing over it.

The thing is, this 400 km. would be FLAT, according to Google maps. No mountain ranges except for the islands in the middle. I would expect the wind would scour it like the wind scours the prairies. There would be a constant replacement of the air, like any other prairie or desert area.

The land would also be extremely fertile, with abundant moisture. All of the existing sea life would have settled to the bottom as the water is pumped out. There is no where for it to go. There would be a thick layer of organic matter. I suspect plant and animal life would be plentiful. It would be an almost perfect growing environment, temperature wise.

With some clever landscaping, the wall/dam would look just like any other mountain range on earth surrounding a huge plane with a few other mountains at the center. Think Death Valley with a mountain in the middle, except with an abundance of water and lush vegetation.

EDIT Addendum

What's been stuck in my mind is WHY would anyone do this, what 'problem' is it the 'solution' to? I have come up with an answer.

For some story-related reason, the Earth's atmosphere has been partially blown away - say, a severe solar storm. Something that is allegedly happened on Mars. In order to save any semblance of normal living, a coffer dam is built around Hawaii, and perhaps several other areas of the oceans, such that the remaining atmosphere would concentrate in these 'saucers' sufficiently that the remaining air pressure would be around our current one atmosphere. It would explain why the residents do not try to venture out - the atmosphere density and pressure would not be viable for sustained living. Also, the radiation would be too high.

This also explains why the atmosphere density and pressure is 'normal' in the enclave. Climbing to the top of the mountains (what would be the current sea level) would be like climbing Mount Everest today. If the residents DID climb the mountain, they would obviously recognize that they were inside a coffer dam, and where the actual sea level was. The mountains are, in point of fact, higher than the sea level and higher than the coffer dam. From the top, one would definitely be able to view what was beyond the coffer dam.

This is covered by the request for a 'work-around to make it work'.

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  • $\begingroup$ Cool. The other responses didn't seem to rule out the possibility of human habitation of the sea floor - I just need to know what sort of sci-fi magic I need to shoehorn into the worldbuilding to make it somewhat believable. Despite the air flow, what effect would the higher atmospheric pressure have in your opinion? $\endgroup$
    – Gwood
    Jun 20, 2021 at 13:21
  • $\begingroup$ I am not sure the higher atmospheric pressure would have any major significance. I think it is a red herring. Mexico city is 7,200 feet ABOVE sea level and they have adjusted adequately. Again, I think other posters are confusing the tremendous WATER pressures with AIR pressures. Therapeutic barometric chambers are three times normal air pressure at 100% oxygen. medicalnewstoday.com/articles/313155#what-to-expect $\endgroup$ Jun 20, 2021 at 14:11
  • $\begingroup$ With a contiguously flowing wind, the atmosphere would be continuously replaced, and I do not see any tremendous difference in oxygen concentration. In typical weather patterns, areas of high pressure are right beside areas of low pressure. I have never heard about oxygen concentration level differences between them. In fact, the higher air pressure may actually PUSH the air out in a continuous air flow, creating interesting wind patterns, similar to those between high and low pressures in normal weather patterns. But the air would be continuously replaced. Again, think 'saucer' not 'canyon'. $\endgroup$ Jun 20, 2021 at 14:21
  • $\begingroup$ Hmm okay. The closest analog to this sort of undertaking I've been able to find is the Atlantropa project en.wikipedia.org/wiki/Atlantropa . . . and according to my understanding, draining the Mediterranean would create an uninhabitable desert in the "canyon" of the former sea. Is the difference here due to the sea floor around Hawaii being a saucer and not a canyon - or does it have to do with time scale or other factors? $\endgroup$
    – Gwood
    Jun 20, 2021 at 14:49
  • $\begingroup$ And exactly how did they propose to lower the Mediterranean Sea, such that it was lower than the Atlantic? Where does the water go? That idea was never more than a poorly-thought-out fairy tale. It assumes the water all evaporates? Essentially, all of the power generated would have to be used to pump the water back out. Damming the Straits of Gibraltar so that the water backs UP in the Mediterranean, now THERE is a thought. $\endgroup$ Jun 20, 2021 at 15:00

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