U-boats were equipped with crude breathing apparatuses consisting of oxygen bottles connected to a hose and mouthpiece. Bare-bones contraptions like these, along with life preservers, gave some crewmembers at least a chance of surviving a sinking craft.

“You’d stick the mouthpiece into your mouth and turn the oxygen on; then you’d open up a hatch, let the compartment flood, and swim out of that flooded compartment,” Bennett said. “But there’d be no guarantee that you’d survive the experience, depending on what depth you’re ascending from.”