Question

How do submarines sink or float inside the surface of water?

Answer 1

A submarine has the ability to float and sink. The ability to control buoyancy comes from the submarine's trim or ballast tanks which can be filled with either water or air, depending on whether the submarine needs to float or sink. When the submarine floats, it means its trim tanks are filled with air.

Answer 2

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$\longmapsto$This is made possible by the large ballast tanks provided in the submarine. To make a submarine sink(or dive),these tanks are filled with water so as to increase its weight. To make a submarine rise to the surface, these tanks are emptied by pumping out water. When water is pumped out of these tanks, the weight of the submarine decreases and the submarine rises up due to the upward buoyant force of the sea water.