system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do submarines go underwater and come back up?

I mean that how do they off set the buoyancy of the air inside the hull and how do they recreate that buoyancy to come back up?
gpt
They are designed to be buoyant (lighter than the weight of the water they displace, up to a certain depth), and they pump in seawater to internal ballast tanks to increase their weight until they start to sink.  To rise, they pump the ballast water back out again.