I’ve been reading quite a lot about ww2 subs. Subs that dived for longer times could have pressures inside different to the outside pressure. This wasn’t enough to cause health issues, but it could make it either difficult to open the hatch or it blew open itself once unlocked.

Related to the pressure thing: When late war German uboats used the Schorchel technology (which is basically just an extended air intake and exhaust, so the submarine could use its diesel machines at periscope depth) in bad weather, this could result in sudden pressure drops due to the schnorchel being submerged under a wave for a short time and the engines sucking the air out of the submarine itself. It was no easy ride

I want to know this as well. This is the only reason why I often switch between browser feddit and Boost.