Submarine

To dive underwater and then resurface, the submarine's crew must be able to control the ship's weight or density. We can compare this to an empty can floating in the water. The empty can could become more "rock-like" and sink by pouring water into the can. By controlling the amount of water we pour into the can, we can control the rate and depth of its dive under the water's surface.

Submarines have an inner and an outer steel shell, called a hull. The area in between the two hulls is called the ballast tank. It can be filled with either air or water. When the submarine is on the surface, the ballast tanks are filled with air and the submarine's overall density is less than that of the surrounding water.To surface, the submarine operator releases compressed air into the ballast tanks. The high-pressure air forces seawater out of the bottom of the tanks until the submarine's overall density is less than the surrounding water. When this happens, it floats upward. This is called positive buoyancy. The hydroplanes are angled upward so the bow (front) of the boat rises first. By controlling the amount of air and water in the ballast tanks, a submarine can operate at any depth under the water.Ballast tanks are open at the bottom. To dive, the submarine operator opens up valves at the top and lets air out. It's something like blowing bubbles when you are swimming underwater. Seawater rushes in to fill the space that was taken up by air. This changes the ship's density.

When the density of the submarine is greater than the surrounding water, it begins to sink. This is called negative buoyancy. A moveable set of wings, called hydroplanes, helps control the angle of the dive.

To keep the submarine at any specific depth, the crew adjusts the mixture of air and water in separate, smaller ballast tanks. The operator tries to keep the submarine's overall density about the same as the surrounding water. This is called neutral buoyancy. When the submarine reaches its cruising depth, the hydroplanes are straightened so the craft can travel level through the water. If something changes the submarine's weight and density, such as firing torpedoes, the operator must make further adjustments of the water/air mixture in the ballast tanks

Source: http://express.howstuffworks.com/express-submarine1.htm

go to the website and Click on submerge to see the submarine sink. Click on surface to see it float.