I don’t know if this is done in practice, but if you have a nuclear powered sub, implementing a water electrolyzer that makes oxygen is fairly trivial. Then you have air as long as you have power, so they could in principle stay submerged for ≈ 20 years, or however long the nuclear reactors can go without refill.
Technically a Sub can stay underwater forever, it is the crew that is the problem there. If they had Star Trek replicators to make them food with that reactor then boredom becomes the limiting factor.
I don’t know if this is done in practice, but if you have a nuclear powered sub, implementing a water electrolyzer that makes oxygen is fairly trivial. Then you have air as long as you have power, so they could in principle stay submerged for ≈ 20 years, or however long the nuclear reactors can go without refill.
Technically a Sub can stay underwater forever, it is the crew that is the problem there. If they had Star Trek replicators to make them food with that reactor then boredom becomes the limiting factor.