why submarine go into water although having displaced large amount of water
Submarines are a mixture of metal (the hull), air, and water (the ballast). The secret of a submarines ability to either sink or float lies in a special property of air. Unlike water or metal, air can be squashed into a tiny space. While the submarine is sinking, its air is compressed. Water fills the compartments called the ballast tanks. The combination of water and metal, with just a little bit of air in the centre for the crew to breathe, is more dense than the surrounding ocean water, and so the submarine sinks.
Once the submarine is underwater, air is pumped into the ballast tanks. The new combination of metal, water, and air is just as dense as the surrounding water, so the submarine hovers, neither sinking nor rising. This is called neutral buoyancy, and allows the sub to maneuver underwater.