Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
how does a submarine go up and down
People picture a submarine diving by tilting its fins, like a plane. That is not the main trick. It dives by getting heavier.
Floating is a tug of war: weight pulls down, and the sea pushes up. Seawater weighs about 1.025 kilograms per litre.
So the boat carries ballast tanks, big chambers in its hull. Full of air, they keep it floating. Fill them with water, and it gets heavier.
To dive, open the vents on top so the air escapes, and let seawater flood in at the bottom. Now heavier, the boat sinks.
To rise, close the vents and blast compressed air into the tanks. It shoves the water out, the boat gets lighter, and it rises.
Small trim tanks at the front and back hold a little water. Pumping it between them keeps the boat level, and balanced weight lets it hover.
Stern fins, called hydroplanes, work like wings, but only while the boat moves. Water rushing over a tilted fin pushes the tail up, tipping the nose down.
So: flood the tanks to sink, blow them with air to rise, trim to stay level, and use the fins to steer while moving.
A submarine never fights the sea to go down or up. It changes its weight, and the water does the rest.