Why does a submarine stay under water?

A submarine stays underwater primarily due to its ability to control buoyancy through ballast tanks that can be filled with water or air. By adjusting the amount of water in these tanks, the submarine can achieve neutral buoyancy, allowing it to sink, float, or remain at a desired depth. Additionally, submarines are designed to withstand the pressure of deep water, enabling them to operate effectively below the surface. This capability allows them to conduct stealth operations and avoid detection.