The Problems With Submarines

We know that submarines can already exist at least approximately at these depths. But why couldn't they just recreate the conditions of a sub but in something that's somehow anchored to the ocean floor? The big question is whether humans can exist for an indefinite time there without running out of air or food and killing each other. Depending on where this base is constructed, like if it's deep underground in some kind of cavern or something, you could escape all kinds of natural disasters,. There are hundreds of things you could get away from just because you're deep inside the earth, inside the water.