How does a submarine stay under water?

You may have noticed that an empty bottle will float along the surface of water, but one which is full of liquid will sink. The submarine works in exactly the same way. Each submarine has several large tanks, which can be filled with either water or air. When they were filled with air, the submarine will float on the surface of the water, just like a ship. When it is time for the submarine to dive, large amounts of water are pumped into the tanks, so making the vessel much heavier. By regulating the amount of water and air in the tanks, the crew of the submarine can make their vessel rise or sink to various levels in the water.