The available pressure at 700m underwater is considerable. There's no equivalent reservoir of high-pressure fluid on the surface (the atmosphere is at a much lo

The available pressure at 700m underwater is considerable. There's no equivalent reservoir of high-pressure fluid on the surface (the atmosphere is at a much lower pressure, obviously). (The pressure difference in submarines is notably much bigger and more troublesome than the pressure difference spacecraft in a vacuum need to resist.) So surely it's the high pressure at depth that's the attractive resource here? Pumping a gas into a cavern could offer a positive pressure solution, and I'm pretty sure that's been proposed already.