Why do submarine not sink?

As with all boats, a submarine does not weigh as much as the amount of water it displaces. To go under the water's surface, it pumps water into special tanks (ballast tanks), and to rise up again those pumps force the water out of the ballast tanks and back into the sea.

Ans 2 - Submarines can go deep and rise again by using water ballast and air pressure, as planned by the captain. -Unfortunately, when things go wrong, submarines DO sink.