Why do submarines dive and surface?

Answer: Ballast tanks fill or empty water

Wings tilt to change depth — Wrong. Submarines use diving planes (fins) for fine depth control when moving, but changing depth primarily uses ballast tanks altering buoyancy.

Ballast tanks fill or empty water ✓ — Correct! Submarines have ballast tanks. To dive, they fill tanks with water, making the sub heavier than the displaced water (negative buoyancy). To surface, compressed air blows water out, making the sub lighter (positive buoyancy). This changes overall density.

Propellers change direction — Wrong. Propellers drive submarines forward/backward, not up/down. Ballast tanks control diving and surfacing by changing buoyancy.