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Suppose we've colonized a planet with 99% of the surface area covered by oceans. It has submerged continental crusts, averaging 200-500m underwater, with the average depth of open ocean being 1000-1500m. Other than the difference in surface geography, consider the planet generally earthlike (unless deviating from Earth-normal conditions is necessary to answer the question).

Colonization was via slower-than-light generation ship, so this colony must be self-sufficient. Natural resources are mined from the rock of the continental shelf, which requires either above-water mining rigs or seafloor bases at shallowest parts of the ocean.

In the real world, surface ships and installations are far, far easier to build and sustain than their underwater equivalents. They don't need to be sealed against intense pressure and they're less mechanically complex to maintain, even if I posit a non-breathable atmosphere to eliminate the advantage of not needing to be sealed at all. But I'm looking for a reason or reasons to justify this colonial civilization being primarily aquatic, living in submarines and undersea installations exclusively, rather than ships and oversized oil rigs.

The best justification I've been able to come up with is extreme weather on the surface. According to this question/answer, a mere 133m depth is sufficient to ride out the worst storms overhead. However, since oil rigs seem able to survive hurricanes as long as they're built tall enough to avoid the worst of the waves, I'm skeptical that all the headache of an underwater city would be more attractive than just building a more storm-resistant above-water installation.

Simply put: Why live underwater, rather than on the surface of the ocean?

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    $\begingroup$ "It has continents, but they average 200-500m underwater". Continents are land. Do you mean continental crust? $\endgroup$ – RonJohn Aug 12 '18 at 17:29
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    $\begingroup$ Yes, thanks for catching that- I'll make an edit. $\endgroup$ – Catgut Aug 12 '18 at 17:30
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    $\begingroup$ On a water world, you're going to have a lot of problems whether above, on or below the surface. $\endgroup$ – RonJohn Aug 12 '18 at 17:30
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    $\begingroup$ @RonJohn In geology continent refers to the crustal formation not whether it's above or below water, underwater continents make perfect sense in that context. $\endgroup$ – Ash Aug 12 '18 at 17:45
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    $\begingroup$ Just make the storms stronger. $\endgroup$ – b.Lorenz Aug 12 '18 at 18:21
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Waves, on Earth wave height is, in large part, constrained by the fact that waves meet land relatively quickly in most of our oceans. On a water world there's very few chances for surface waves to break, so they'll keep building until they reach monstrous heights and swamp habitats floating on the surface. The base of globe circling wave trains will probably scour the surface of the shallower continental shelves as they roll through, you'll want to go deep to maximise the survival chances of any habitat. The static pressures at depth are higher but easier to engineer for than dynamic forces that will tend to cause fatigue. On a water world mineral deposits and stable power sources aren't going to be found on the surface anyway, if you have to go down anyway you may as well go deep.

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    $\begingroup$ I heard that the waves are highest close to land while on the open ocean (say middle of Pacific) there you can feel them much less. Not sure if this is true though. $\endgroup$ – Trilarion Aug 13 '18 at 9:17
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    $\begingroup$ @Trilarion Waves do slow down and mount up in shallower water and then eventually break when the water is shallow enough but on this world that won't happen, they'll just build and build. There is a gravity governed maximum wave height but on Earth it is very rarely reached on a water world it would be the norm. $\endgroup$ – Ash Aug 13 '18 at 9:24
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    $\begingroup$ But the energy for building up and up must come from somewhere. There is also friction between water molecules and so water wants to be clam naturally. Only if you sort of shake it or inject energy another way, you get waves. One way might be a high speed rotation of the planet (like 1 hour per rotation). That would add a lot of energy into the system which might result in gigantic monster waves. $\endgroup$ – Trilarion Aug 13 '18 at 9:30
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    $\begingroup$ Not sure if the absence of continents really would induce large waves. Over the Pacific you surely have thousands of kilometers of water surface without land and people don't really report monster waves that often. Waves also dissipate. For a significant build up you usually need some kind of resonant effect, for example the tidal waves travel with approximately one rotation per day? There are probably scientific simulations how large the waves would be on Earth without continents. $\endgroup$ – Trilarion Aug 13 '18 at 10:19
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    $\begingroup$ @Ash and why do you think that? the Atlantic has half the globe to build up waves, and at most gets 23m. While there's no land to dissipate the energy, winds going in the other direction will. Ultimately, the waves aren't going to be bigger. In addition any seismic activity that causes pressure waves in the water will just destroy anything underwater with no warning. In addition, if things go wrong for any reason, the colony will be dead. It's why we use ships to transport goods rather than subs. $\endgroup$ – UKMonkey Aug 13 '18 at 13:55
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The best reason to build submerged cities is the same reason to build subterranean cities; protection from surface threats. Specifically radiation.

Water provides amazing protection from radiation. Nuclear powerplants store their recently-spent fuel in big pools while it cools off. Water is so good at stopping radiation, divers routinely do maintenance on them without removing the spent fuel rods. Water cuts radiation in half approximately every 7 centimeters, so as long as they don't dive right down and touch the spent fuel rods, they actually receive less radiation than you do just from the natural background. https://what-if.xkcd.com/29/

So if this planet has just had a nuclear war, especially one using cobalt bombs which produce much more dangerous fall-out, submerged cities will be the best place to live. The ocean will block most atmospheric radiation, and the currents will keep too much fall-out from piling up on the city's hull.

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  • $\begingroup$ Upon arrival they may also discover the planet has a highly radioactive moon, or their sun itself is highly radioactive. $\endgroup$ – Amadeus Aug 12 '18 at 20:08
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    $\begingroup$ @Ryan_L the radiation could come just from their planet missing a magnetosphere, since that plays a big part of protecting earth from solar radiation. $\endgroup$ – Jack Aug 13 '18 at 1:04
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    $\begingroup$ The solar radiation/missing magnetosphere version works a lot better than nuclear fallout here. Cobalt-60 has a half-life of 5 years, so it will quite likely decay away to manageable levels in less time than it takes to build a submarine city. More broadly, if you have the technology to withstand 10 atms pressure and provide air underwater, you have the technology to build a surface-based fallout shelter for a lot less trouble. $\endgroup$ – Geoffrey Brent Aug 13 '18 at 4:07
  • $\begingroup$ Radiation and (potential, if it fits thematically) strange alien beasts were the first thoughts that came to mind as well. $\endgroup$ – dimwittedanimal Aug 13 '18 at 18:59
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    $\begingroup$ Radioactive and radiant are two different things. If there is a strong source of X-rays in the sky (such as a nearby pulsar), or the sun is a searing ultraviolet horror, then yes, I would prefer to live a few meters below the surface. But the simplest and safest design for that would be a nearly-neutral structure hanging under a large buoy, which might not be what the OP is looking for. $\endgroup$ – Beta Aug 13 '18 at 20:12
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List of conditions that would make living on surface not so tempting idea:

1) Small planet

  • Pressure clearly lower than 1 atm, so in order to replicate earth conditions, actually should be pressurized.

  • This thin atmosphere does not have enough of ozone, thus extra protection against UV is needed anyway

  • Lower gravity means slower pressure increase with depth

2) Flare star

  • Luckily not as strong to strip atmosphere but enough to boil the surface of ocean from time to time.

3) Very elliptical planet orbit

  • the planet surface temperature on average is comfortable +10 C. Just it means -40 aphelion (sort of winter) and +60 in perihelion (sort of summer).
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As always, I believe the reason would be economic.

200-500m deep is not that deep compared to many parts of the ocean on earth, and let's say your shallower areas have valuable resources on the ocean floor (such as lithium deposits, platinum or titanium), or heat sources for use in power plants (such as thermal vents).

If we have already journeyed to this planet, then we already have ready knowledge of life support and habitat systems independent of environment. It would be easy and cheap to use these systems in underwater environments, but also making use of underwater power sources and minerals.

Consider too that ocean wide planets would have unusual weather systems, possibly a high degree of moisture in the air. Meaning it may be quite overcast all the time, dark and gloomy, and windy. Not a great place to enjoy a vacation. Instead, underwater habitats may be much more stable, also with an abundance of amazing marine life, requiring constant study yet also fascination.

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    $\begingroup$ Note that underwater life concentrates 10 to 200 meters deep, and mostly 10 to 50. A planet without the shallows would have much less life than Earth. But weather systems could indeed be very different. $\endgroup$ – Therac Aug 12 '18 at 19:05
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Size, space, the room to breathe, and the necessity of dirt. They don't just want to be underwater (for protection from storms and radiation), they actually want to be in million-strong cities on land, they want space. On the ocean floor they can burrow to create underground caverns miles across and a mile deep, so they can have industrial zones, residential zones, business zones, government zones, various recreation zones, and farming and ranch zones, just like we do on Earth. With advanced scifi tech, this can all be fusion powered (fusion being performed in its own safe power generation zone), and lit up like daylight. With enough power, you could have a cold zone in there, for skiing or other snow sports.

The only thing you really need access to the actual sky for is astronomy and security for look out or receiving remote signals; that could all be done on ships, in person or remotely piloted. The ships and submarines can dock at the port to the underground city; which can be more like the size of one of our stadiums; not ten times the size of New York City like the actual underground city.

Underground their robot farms and ranches grow straight up Earth food, cucumbers and lettuce, apples and oranges and bananas. Chickens and eggs, butter and milk, if they haven't gone vegan. They've got robotic bees with photo-electric wings to gather power, for pollinations.

Even if they don't burrow in; it would be easier and more efficient and safer to maintain a thousand closely spaced domes underwater on the ground than to try and do the same thing with floating ships, subject to storms and waves.

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Give it a hot, uninhabitable atmosphere (like the one of Venus).

Venus athmosphere consists of 96% CO2 at a pressure of 90 bar and a surface temperature of 460 deg. Celsius. Not a very nice place to live.

The conditions on Venus are powered by closer proximity to the Sun and the greenhouse effect of the CO2 (we are working to get there ourselves).

Maybe you want it not that bad with a lower surface temperature (below boiling point of water for sure) and generally more water.

However, even then it might not be very difficult to see that conditions below ocean surface are preferable to those above.

How could it have happened? Sudden increase in solar activity leading to increased radiation exposure or sudden increase in seismic activity coupled with volcanic activity and huge CO2 concentration in the atmosphere and subsequent increase in temperature or a collision with another object (moon-like or smaller).

That might also explain why you colonists thought it would be a good idea to go there. It all looked so good until they arrived and realized what has happened in the mean time. But no time for despair. Time to roll up the sleeves and get to work to build an under water home.

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    $\begingroup$ There would be no liquid water anywhere on a planet that hot. $\endgroup$ – Ash Aug 13 '18 at 9:26
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    $\begingroup$ @Ash That's why I wrote that you probably do not want it not that bad with a lower surface temperature. It's extreme, but it shows the direction. $\endgroup$ – Trilarion Aug 13 '18 at 9:27
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Wind. Lots of it. Driving enormous waves, too.

What would hurricanes be like on a world whose surface is mostly water?

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It’s pretty simple. All the resources are at the bottom of the ocean, including energy (oil, coal, thermal vents), building materials (stone, iron, copper), precious ores (gold, diamonds), and food (plants and fish). The primary need of any civilization will be these resources. It would make sense for civilization to start near those resources. Any industries of this planet would be at the bottom of the ocean.

If you're starting a mining rig on a gold vein, and you need energy, are you going to want to pump oil all the way to the surface, then all the way back down to the mine? Is everyone who works in that mine and that oil rig going to want to take a long commute every morning to work if they can avoid it? Of course not. It's much cheaper and easier to live near the mine and the oil rig and have a short pipe. Oil rigs on Earth are built on the surface because that's where the end customer is anyways. When the majority of customers are at the ocean bottom, it's far cheaper to pipe oil directly to them.

There’s nothing on the surface except water. If the surface is bleak, uninteresting, uninviting, dangerous, etc... it’s hard to see why anyone would choose to travel or live there in the first place. Once you get the infrastructure in place, you can build underwater tunnels and buildings pretty effectively, and any space-baring civilization would have this kind of thing mastered pretty well.

Just in case, it may not hurt to have:

  • Blistering radiation.
  • Awful storms.
  • A few horror stories of foolish people who made a go of the surface.
  • A nice retirement planet nearby.

People come to your planet to work and make a fortune, then go off to retire somewhere else.

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The planet could be undergoing a meteor bombardment phase where largely water ice, CO2 ice or methane ice meteors pummel the surface of the ocean. Their destruction from energy of impact might mean that they don't pose threat to submarine cities (But tsunamis and rogue waves on surface)

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