Ballast tanks don't work at these depths because of the extreme pressure, which is why all these deep sea vehicles are submersibles rather than submarines. In

Ballast tanks don't work at these depths because of the extreme pressure, which is why all these deep sea vehicles are submersibles rather than submarines. In fact I think if a submarine goes below a certain depth then it can't adjust its buoyancy to return to the surface again, but I am also not a sub nerd.