Depends on which vents are opened, some go to Ballast tanks, some go to different valves throughout the Boat for trim purposes, some are for pumps, etc. usually (if its the outside vents You're speaking of) when they are inspected You see a dark hole, shining a light inside just shows a long pipe inside... I got to see a U-Boat while I was in Hawaii, dont remember which one, but a dude was there to answer questions, and He explained the wherefores and How to's about it, that was one topic alot of people asked to be explained, there was also an American Sub there called the Bowfin, The U-Boat was so much more advanced than the US Sub it was Uncanny. Have a Good One!
KIP