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I have made my billions here in the present day, and I'd like to have a permanent underwater settlement.

It is about as useless as the Mars One mission (yes, I went there), but some people may come and go as they like or can afford to.

This is fully pressurized and connection with the outside world is similar to a space station, whereby airlocks and submersibles connect you to the outside world, but at this location there's still some light and warmth. This is only a depth of 50m in the tropics - pretty far away from the island nation of Nauru, so not a lot nearby. Plopped right on the ocean floor, and yeah, I got my environmental permits.

I'd like to house 1,000 people, of which 1/4 are research-related (6 months rotating), 1/2 are tourists (1 month, rotating), and the remaining are staff and I have even received express interest in full-time retirees to live 'on board'. So there has to be additional space for study, leisure, housing, etc.

What are my opportunities and constraints for construction, power, water, wastewater, communications, transport, air, trash, etc.? I'm not interested in cost, but a magnitude of order would be helpful. Any other ideas are helpful, particularly design considerations.

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  • $\begingroup$ How self-sufficient does this city have to be? The ISS has to get regular supply runs, whereas Mars One would have to be self-sufficient, at least in the short-term. Given you're cycling tourists at least once per month, I suspect you'll be importing a lot of supplies and exporting a fair amount of trash. $\endgroup$ – Martin Carney Apr 18 '15 at 16:22
  • $\begingroup$ Point of clarification: Is the entire city going to be pressurized, as you say, so that coming and going will require decompression or will it be at 1 atm, making decompression unnecessary but construction possibly more difficult? $\endgroup$ – Scott Baker Apr 18 '15 at 18:41
  • $\begingroup$ It will be at 1atm. $\endgroup$ – Mikey Apr 18 '15 at 19:53
  • $\begingroup$ Evacuation should a deadly Ebola virus spread or seaquakes, seiche waves etc. $\endgroup$ – user6760 Apr 19 '15 at 4:45
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Someone is trying to do something like this on a smaller scale.
But you could do better.

Construction
Thankfully 50 meters isn't all that far down, so while it will take some heavy duty construction, it won't take Rapture level technology.

Power
You could use a few different renewable power technologies. Wind is easy. Tidal power might be an option if there is land somewhat close. Biogas could be generated on site. There has been work on using algae to produce fuel. Hydrogen could be produced by electrolysing water from the Water Recovery System to produce oxygen and hydrogen.

Water
Desalination, evaporation, and recycling are going to be your best options. Desalination and evaporation could be combined with the power generation if you go with biogas or algae fuel. Keeping it self contained like the ISS will help as then you'll only need to replace a little instead of a lot.

Wastewater
Use the same methods used on the ISS. You want to replace as little water as possible.

Communications
Having a radio buoy would be an easy way, and could easily include satellite too. Just raise it on a wire, pull it down in really bad weather. Keep spares.

Transport
Submersible would be one good option, but you could have a floating platform with a elevator tube as well. This would allow easier coming and going, and help with communication and air. Plus it would give you a place to dock boats.

Air
Similar to the ISS again. Electrolysis from the water reclamation system, pulled from the surface, or recycled in a green house.

Trash
This would be the hardest one. You'd want to remove as much of it as possible before it got to your city. No extra packaging, make as many things as possible biodegradable so they can be composted, recycle as much as you can.

Food
A few greenhouses are going to be important for food, fuel (algea), air, water filtration, sanity, etc.
I'm sure fish will be another big part.
Probably import some stuff, but you'd want to be as self sufficient as possible.

Windows
If you can't have windows then what's the point If being under water in the first place? Pressure is an issue though. As WhatRoughBeast points out, big windows are probably impractical, though with the right laminated materials and with enough thickness it's probably doable. The other option is to have smaller windows, but more of them.
The window on this is designed to go down to 300m:
underwater vehicle
So you get a lot of them and put them all over. Being stuck in a tin can 50m under water without being able to look out for any length off time would be a horrible vacation.

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  • $\begingroup$ Greenhouses aren't a great idea. At 50 meters down, you're not in the Great Dark, more like the Great Twilight, but you certainly don't have enough light to grow anything, even if you were willing to take the risk of big windows. Attempting to grow food with artificial illumination is grotesquely expensive. $\endgroup$ – WhatRoughBeast Apr 18 '15 at 16:01
  • $\begingroup$ @WhatRoughBeast you can get low energy grow lights, and either way it's still going to be cheaper than importing it all out to the middle of the ocean, especially for fresh produce. $\endgroup$ – AndyD273 Apr 18 '15 at 22:49
  • $\begingroup$ Do you have any numbers to support this? Assume 500 watts/m^2, 25% LED efficiency,16 hrs/day, 20 m^2 / person, 1000 people, power cost $1.00/kwh. Veggies cost $64,000 per day to grow in power alone. If weekly delivery of produce costs less than $400,000, it's cheaper than grow your own. $\endgroup$ – WhatRoughBeast Apr 19 '15 at 0:44
  • $\begingroup$ @WhatRoughBeast 1)At 50m you still get 60%-70% of the surface. So you only need to supplement. 2)vertical hydroponics will get you a lot more produce for the same amount of space, which cuts down on the lights needed. 3)If you can afford to vacation here, $50 for a salad is reasonable. 4)The electricity is coming from renewable sources, so it's not 1.00kwh. The greenhouses are also helping grow fuel (algae/biogas). 5)They are part of the air/water filtration system, so the cost is defrayed. 6)Don't underestimate the power of greenery on sanity. 7)Some stuff will be imported anyway. Just less. $\endgroup$ – AndyD273 Apr 19 '15 at 2:25
  • $\begingroup$ @AndyD723 - responses: 1) 10 m^2 x 1000 is 2000 m^2. Big windows at 50 meters? Not likely. 2) vertical hydroponics reduces floor space, not light requirements. 3) Only half are tourists - how much do you think hotel staff get paid? 4) Renewable is not cheap. Small plant means high capital costs, low efficiency, high price. And more biomass produced means more space and light required. Bigger windows. 5) Irrelevant. 6) Irrelevant. 7) Change of tune from "as self-sufficient as possible". Plus, sending a cargo ship half-full costs almost as much as a full one., $\endgroup$ – WhatRoughBeast Apr 19 '15 at 3:38
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Communication

Seawater is highly resistive to radio waves at the frequencies we usually use them. (see page 7, section Sea Water.) However, at lower frequencies, 10-30 kHz, it is entirely possible. Unfortunately, this frequency allocation chart shows that the entire 10-30 kHz range is occupied. You'd be interfering with other signals, if you're using the same systems. However, you could set things up in your world so that the 10-30 kHz range is completely empty, at which point you can put radio communications from your city in it.

Your other option, since you're not that far underwater, is to build a big antenna extending above the surface, which you could then assign any frequency to since air conducts radio transmissions.

The alternative, which you can use for many of these sections, is a tunnel leading out that takes you to the surface.

Transport

Transport around your city is not a problem - since you're pressurising the whole area, you can just use regular methods of transport.

To get in and out of your city, your setup of airlocks and submersibles is sufficient, but you might want to consider big submersibles for that many people. The alternative, as I alluded to, is to use your communications tunnel to move people as well.

Power

This answer is going to get very boring very quickly if I just keep saying "use the tunnel". However, you could do, just run a big cable down it. The alternative, which is slightly more interesting, is to build an osmotic power plant or salinity gradient power plant, which uses osmosis between and fresh water source and a saline water source to create power. Assuming you also have a desalination plant set up for water, you have both required water types.

Air

Run a pair of tubes up to the surface and stick big pumps on both of them. One of them should pull air out of the underwater dome and one should put air from outside back into it. You could also add some filters if you want really clean air - this might also work as a health centre where people with weak immune systems could come to because you've cleaned the air. Just make sure you decontaminate every visitor.

Waste

You need transport for this, basically. The simple solution is to take bags of waste out on every submersible trip and bring in supplies. You could also have some fun by installing a flexible tube leading to the surface and putting some targets around on the surface... trash bag shooting becomes your new sport. Shoot them with air pressure.

For waste water, do what every country does with their waste water already: recycle it. Clean it out, filter it, sanitize it, and put it back into the fresh water system. (If this disgusts you, don't worry - you're drinking the water that someone flushed down the toilet a month or two ago.) You can dump the remaining solid waste into the sea. Again, if you protest violently against this, countries already do it and have done for a very long time.


See also eharper256's comment below this post explaining some alternatives.

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    $\begingroup$ I don't much anything to add to this from apart from: Osmotic power might eventually destroy the local environment due to the change in salinity (osmotic power is ideally built at the end of rivers normally where there is natural fluctuation). A couple of Tidal Barrage towers built up from the sea bed could work as alternatives. Additionally, you can burn your solid waste for power with Anaerobic digestion techniques. $\endgroup$ – eharper256 Apr 18 '15 at 14:34
  • $\begingroup$ @eharper256 See my edit $\endgroup$ – ArtOfCode Apr 18 '15 at 14:35
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    $\begingroup$ For Power, remember that you can only get energy out of a system if it has already been put in. Splitting water into H and O, for example, requires an energy source, and you'll get less energy out of burning the H and O than you put into separating them. Same problem with desalinization plus osmotic power - you'll put more energy into desalinization than you'll get out of an osmotic generator. Tidal power could work. You could float solar panels on the surface above your city. Or float some sort of wind power. $\endgroup$ – Martin Carney Apr 18 '15 at 16:17
  • $\begingroup$ If we had stable Hydrogen fusion reactors, you could split water and fuse the H atoms into He to generate energy. But that technology is still in the future, and you specified here in the present day as a constraint. $\endgroup$ – Martin Carney Apr 18 '15 at 16:20
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    $\begingroup$ Solid wastes from human excrement could be recycled into humanure and used to grow plants. It would be a fascinating experiment to see just how trash-free you could get this environment to be. $\endgroup$ – Scott Baker Apr 18 '15 at 18:43
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You've got a big air supply problem. Underwater habitats are normally pressurized to ambient but that's got some big headaches:

The lesser problem is the rapture of the deep. Everyone is going to go around a bit drunk all the time. The real problem, though, is the oxygen. Your ambient pressure at that depth is 87 psi. * 21% oxygen in ambient air = 18.27 psi of oxygen. Your safety limit for this is only 3 hours.

You're going to need a fancy atmosphere plant--use an oxygen concentrator to pull some of the oxygen out of the air before you pump it into your dome.

Also, decompression from saturation at this depth takes 39 hours if it's done continuously and in practice it will take 2 1/2 days as it's not normally done during sleeping time.

The alternative is to maintain the habitat at surface pressure but that makes for big hassles going in and out to the surrounding water--the main reason for an underwater habitat is the free passage between the living space and the outside.

There's a reason the previous attempts in this direction have been set shallower!

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Have a look at how submarines work, that should give you a good idea.

In a flight of fancy I am going to, with my limited knowledge speculate what might be required.

You should address power since most other things can be solved by conventional means when that is satisfied. If you have power you have artificial light that you can use to grow food and scrub the atmosphere of CO2. Biological waste can be incinerated with heat to release CO2 for the plants. The minerals can be recovered from the ash. Desalinating water is a solved problem so I am not even going to address that.

For power you have two options that are not very far fetched, geothermal in the form of hydrothermal vents and nuclear. With hydrothermal vents your location is bound, with nuclear it is not. In both cases you could use conventional methods of power generation. Heat $\to$ steam $\to$ electricity.

Nuclear can be done the way that submarines work, so that is solved. I am not sure if self-sufficiency is feasible with nuclear, since you require enriched uranium. That being said, fuel by weigh is minuscule compared to coal. To be clear, the ocean is abundant with uranium, just not the type you can use for a nuclear reactor.

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Dome needs to be highly strong to support 50m of water, density a ton per metre cubed. That's 50 tonnes of pressure on every square metre of dome. Fortunately, dome is a strong shape and should support the weight if you make it reasonably thick - maybe some inches to a foot.

You can just use pipes for most of the other problems. Waste, air, water, can be taken in and out using pipes. People need the submersibels but thats the only thing they'd be used for.

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Material

You need thick glass or some other material to support 50 tonnes on every square meter. Thickness depends on how safe you want the settlement to be: thicker glass is less likely to fail under extreme conditions.

Transport

You can just use the same method of transport to get everything, waste, water, food, supplies, people, in and out. I suggest just using big submersibles: if you have enough capacity, you can get a good load of people and all the other necessities in or out in one load.

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