How a submarine changes depth
A submarine manages weight and buoyancy to travel underwater.

Picture in words: 1. Floating: At the surface, upward buoyancy balances the submarine’s weight.
2. Ballast tanks: Letting water replace air in ballast tanks increases the submarine’s total mass.
3. Diving: Changing buoyancy helps it submerge. Moving submarines also use control surfaces to change depth.
4. Rising: Compressed air pushes water out of the main ballast tanks, increasing buoyancy relative to weight.

One more wonder: The crew stays inside a strong pressure hull, not inside the ballast tanks.


Question: What enters the main ballast tanks to help a submarine dive?
Answer: Water

Sources:
US Navy Museum: submarine buoyancy — https://www.history.navy.mil/get-involved/for-educators/lesson-plans/elementary-school-lesson-plans/history-of-submarines/build-your-own-submarine.html

Reviewed 28 September 2026. Simplified educational illustration, not to scale.
https://atherverse.com/kids/how-things-work/submarines/