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Im sure some are familiar with Bioshock's "Rapture City," a (somewhat) successful underwater city that was able to maintain stability and house an entire population. But, using current technology and engineering, could we (and how) construct a city off the west coast of the United States that:

  1. Is located on the side of an underwater ravine.
  2. Has a max depth of 400 meters
  3. Can inhabit max of 30,000 people
  4. Has a source of food and water (preferably not imported from the mainland)
  5. Adequate defense systems
  6. Standard city departments (Education, law enforcement, public works, etc.)

    Keep in mind, however unrealistic, that budget is practically unlimited, but stay reasonable.

NOTE: If any further clarification is needed, let me know. The more critique,the better

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  • $\begingroup$ If nobody feels like attacking your city, then pretty much any defense systems are adequate. But if you pick a fight with superpowers, you're probably in trouble. $\endgroup$ – Molag Bal May 26 '16 at 2:08
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    $\begingroup$ This world, luckily, doesn't have superpowers. I mentioned the defense system because I considered the rather "wet" environment to have an affect on attack/defense of the city @armadillo $\endgroup$ – Nathan Hardy May 26 '16 at 2:15
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    $\begingroup$ @NathanHardy I would say that it would not be possible, simply because the economics would require a super power to build. At 400m, you need to withstand ~40ATM of pressure. In space, you only really need to withstand ~1ATM of pressure. This is structurally 40 times harder than building the ISS, and that took the combined resources of the international community to build. $\endgroup$ – Aron May 26 '16 at 4:31
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    $\begingroup$ @Aron: I think you're missing why exactly a space station is so difficult to build maintain: because getting there is hard and getting heavy things in orbit is enormously expensive. In an underwater structure you don't have to worry about weight and transporting costs are about as low as they get. $\endgroup$ – fgysin May 26 '16 at 6:08
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    $\begingroup$ I think it is technically possible to build an underwater city, but economically speaking, even if there is a shortage of land for whatever reason (natural increase of population or flooding due to global warming?), it would be much more cost effect to build a city floating on the sea then underwater. At least you don't need to handle the water pressure and oxygen flow. $\endgroup$ – cytsunny May 26 '16 at 9:07
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Yes, this is completely possible today. In fact we already have large elements of it.

We have submarines and submersibles, we have diving gear. There are underwater hotels.

You have two basic choices for the buildings:

  1. You can be linked to the surface air and have routes to the surface. This has the advantage of letting you enter and leave while being dry and cheaper air circulation. Your buildings need to be strong enough to withstand the water pressure though.

  2. You can pressurize the buildings to the same level as the water and just have open pools for access in and out. This has the advantage of letting you access the water around you from the buildings without needing airlocks but you need to pump down and pressurize all of your air. Anyone heading to the surface would spend a LOT of time in decompression.

In practice there would most likely be a combination of these, with buildings at the top linked to the surface and others deeper open to the water. Transition between these buildings could be done using air locks to equalize the pressure.

Note that the depth you mention (400m) is just inside the range of human survival. The records for deep diving at external pressure are around 432m. In practice you would most likely need most of your city higher (and at less pressure) than that as we just have no idea what the long term effects of living in those conditions would be.

The problems here aren't technical. They're practical and financial. Would you want to live in a steel can underwater breathing recirculating and pressurized air? Especially when doing so would be substantially more expensive than living on land.

We have the technology today to build this. What we don't have is the reason to do so.

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It's technically possible but not financially feasible but I see your budget is practically unlimited.

And it would always need power to generate oxygen from some type of water source. The hydrogen might be used as fuel also. Power would also be needed to make freshwater from sea water.

Any metal parts exposed to seawater would need constant upkeep as salt water is very hard on even the best metal parts we have today.

Any moving parts and water intakes and output tubes would need constant upkeep to keep barnacles and other creatures off them. Barnacles, algae, and seaweed are HUGE problems for water intake tubes today, especially near areas where nitrogen-rich runoff goes right into the ocean. Algae is less of a problem in deeper areas where there isn't much sunlight but barnacles and other "fixed" creatures like coral tend to be a problem at many depths.

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