A submarine dives by making itself heavier so that it sinks. It surfaces again by making itself lighter. Submarine use large tanks called buoyancy tanks. When the submarine is on the surface, these tanks are full of air. To make the submarine dive, the tanks are flooded with seawater, making the submarine heavy enough to sink. To make the submarine surface again, compressed air is pumped into the tanks, forcing the water out. This makes the submarine lighter and it floats to the surface. When submarines are underwater, they move up and down using tiny wings called hydroplanes. These work like rudders to strong hulls to prevent them from being crushed by the huge pressure under the water. As submarines dive, the weight of the water pressing down on them becomes greater and greater. You can see how this works by making this model.
Parts of a submarine
A modern submarine, such as this nuclear ballistic missile submarine, is almost as long as a football field - 100yd. It has an engine and propeller at the stern, and is operated by a crew of 140. Steam drives the turbines that turn the propeller. A submarine's hull is strong, but few submarines can go below 1,600ft.
Large plastic bottle, sand, plastic funnel, tank of water, two small plastic bottles, bradawl, scissors, ruler, two plastic drinking straws, rubber bands, nonhardening modeling material, two bulldog clips.