"The early submarines were surface vessels which were able to dive for a few hours at a time, using batteries and electric motors whilst underwater.
The crew just breathed the air inside the sub during that time, and of course it started to get foul with carbon dioxide as they used up the oxygen in that air, eventually forcing the sub to the surface so they could open the hatches to get fresh air into the boat.
Running the diesels brought fresh air in though inlet pipes in the conning tower (sail) too."
Who's the cat that won't cop out when there's danger all about?