The parts of submarine shown in cross-section in the image are:

1. Rudder
2. Propeller
3. Rear diving planes angled upward
4. Forward diving planes angled downward
5. Air being blown out
6. Vent
7. Compressed air tank
8. Outer hull
9. Inner hull
10. Ballast tanks flooding
11. Water being taken as submarine dives in is shown with two thick arrows

The diagram also describes the buoyant forces acting on the submarine:

• Negative buoyancy: As long as it remains submerged, a submarine experiences a constant buoyant force. To overcome the buoyant force so the submarine can dive—that is, to achieve negative buoyancy—its weight must increase. This is done by flooding the ballast tanks, replacing air with water.
• Neutral buoyancy: Once the submarine’s ballast tanks hold enough water for the submarine’s weight to equal the buoyant force, the submarine is said to have neutral buoyancy. It now has no tendency to rise or sink.
• Positive buoyancy: To rise again, the submarine’s weight must be decreased. Air is pumped into the ballast tanks to force the water out. The buoyant force acting on the submarine now exceeds the submarine’s weight. With positive buoyancy, the submarine rises.