@Kampfarsch He isn't specifically storing air in it. That extra space is made with magic, it probably doesn't just immediately suck in that much air from the atmosphere whenever you open the thing and it's not "full" of air. If anything, the bullshit here is being able to survive that long in there without having issues breathing sooner. But that is the kind of game system you'd put so somebody has a chance to try to escape almost certain doom.