Could you explain why being covered with a massive amount of material would save you as opposed to make things worse? Wouldn’t the weight of the material be added to the water’s pressure?
I expect that it would help to be covered by material but I don’t know the physics principles at play which would have that effect, being a liberal arts know-everything-and-nothing.
Well, you certainly need a very thick wall of very strong material around you.
OTOH, whether or not it’s practical to build a habitat under the sea floor is entirely dependent on the strength of material that makes up sea floor. But it might be that such a habitat is more vulnerable to shifts in the earth’s crust.
I think the relative safety of “on” or “below” the seafloor will have to be judged based on the properties of whatever materials would be used for the construction.
I have no idea what the seafloor is made of, but even assuming that it’s just mud a mile deep, perhaps mud’s greater viscosity would make it safer as a surrounding medium than direct exposure to the water? If that makes sense.
I’m not an expert but this makes no sense to me. Whatever the material is it will have to be extremely rigid because it will have to support what amounts to a giant air bubble against all the weight around and above it.
If it’s thick mud, I’m picturing it as oozing through a tiny wall breach and not creating an immediate & cataclysmic high powered jet of salt water.
Also (as a WAG) there’s a limit to how much the mud could be compressed by the water overhead, and therefore a limit to how much pressure it could exert on the habitat, right? – at least I’m guessing that’s so. Could a deep-mud structure under the seafloor thus be subject to less pressure than it would be if it weren’t buried?
I suspect that mud that can ooze will… shriek… under 380 atmospheres. My WAG would be that any material around the habitat will either add to the force resisting the pressure, or add to the pressure against the habitat. I can’t imagine any fluid doing the former.
But maybe someone else who’s more knowledgeable can come along with a better answer.
The mud is going to transition from a point where it is still fluid to a point where it compresses into sedimentary rock. Getting down to the hard rock, which I believe is basalt, would be where you find strong enough material to form a habitat. This is why I pointed out the approach of building an island in the post you didn’t like.
No, I didn’t care for what sounded to me like “I don’t know why the OP is interested in something so pointless.” But if I misunderstood I apologize.
I’ll raise you these ten seconds from Scanners.
Well you got that part right. I could have been more tactful though. The purpose of these habitats would affect their construction. For instance, if you don’t need access to the deep ocean, it does seem a little pointless to be locating a habitat there. But if you have some more specific intent, it would make it easier to address your OP and the followup questions.
Well, there was no real reason, except prestige, to go to the moon when the decision was made. I think doing it (or trying to do it) “because it’s there” is a perfectly acceptable reason assuming it doesn’t destroy the economy in the process.
It would make sense to build lunar base underground to protect against meteorites, which would also reduce the problem of building light-weight yet sturdy materials. Also, if we assume most building to be done on the moon itself by mining the moon and/or nearby asteroids/comets/ meteors, then you already have a hole in the ground.
There still remain the basic problems of making the base self-sufficient (producing food, oxygen and water, dealing with waste) and dealing with atmosphere problems.
I also think that for a long-term colony you would need at least one dome above-ground to see sky and get sun for psychological health of crew, along with space-suits so people can take walks outside (not only for repairs, just to stroll around). I think that artifical full-spectrum lights and videos of outside or VR (the prototype of holodecks) only go so far.
On thinking about it some more, that might not be necessary. You could close the outer lock, open a very small port to the interior to equalize the pressure, then pump all of the water in the lock into a storage pool inside the habitat. To go back out, you’d close the inner lock, re-fill the lock from the storage pool, open a small port to the outside to equalize the pressure, and then open the outer lock.
Ok, I can see this as possible. The problem I can imagine is not being able, ever to get rid of surplus water. If you refill the lock from the pool. and leave any space whatsoever (an air bubble at the top), when the outer door is opened the pressure will compress that bubble and there will be slightly more water after each cycle.
But now I’m wondering how you let the water into the lock in any fashion at all without it being destructive. How do you restrain water with all that pressure behind it to a relative trickle rather than a figurative knife blade stream that eats at the mechanism with every cycle?
After some quick googling I found an off the shelf pump that can deliver 1680 bar, which is well beyond the pressure at that depth.
I stand corrected.
16 gpm too! That’s way better than I expected. Too bad I changed the sump pump recently, but it’s time to follow my own rule and have the next one on the shelf ready to go 
The U.S. Navy has had three Sealab experimental habitats on the ocean floor, although the deepest attempt was only about 600 feet.
After thinking some more, most hydraulic systems today operate somewhere between 150-400 bar, so in terms of moving fluids around, the pressure at 12000 feet isn’t extreme. Since water has a very low compressibility(back of the envelope tell me about <2% at 380 bar) you don’t have to move a lot of it to equalize the pressure. You could fill the air lock at a low pressure and then, like Chronos said, (gradually)open a small port to equalize the pressure.
But radiation sickness is forever, so bury it deep.