system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how does submarines sink when its full of air?
gpt
The hull is very heavy and so they just barely float normally.

They have big chambers inside called ballasts that are filled with air when they are at the surface, but when they want to sink they let seawater into them and pump the air that was in them in to compressed tanks. Compressed air is more dense so it effectively makes the entire vessel less buoyant, making it sink. When they want to rise they decompress the air in the tanks back in to the ballast, pushing the water back out to the ocean.