A submarine is a type of watercraft which is designed to float on the surface of the area and to navigate beneath the surface of the ocean. Submarines range in size from very small submersibles designed for deep sea exploration to huge nuclear submarines maintained as part of a military fleet. All submarines operate on the same basic principles, however, using the property of buoyancy to rise and fall in the water. The earliest submarine was built in the 1600s, although it was extremely crude and bears little resemblance to its modern counterparts.

The double hulled design of a submarine includes large spaces for ballast tanks. When the tanks are filled with air, the submarine is much lighter than the water it displaces as it moves through the water, allowing it to float on the surface. When air is vented out so that water pours in, however, the submarine becomes heavier and starts to sink. The exact depth of the submarine can be controlled by admitting more water or forcing pressurized air into the ballast tanks.

The power plant of a submarine is designed to be highly efficient, so that the craft can navigate for months at a time. Diesel submarines use diesel fuel connected to battery systems to power themselves. A nuclear submarine has a small nuclear power plant. By using nuclear power, the submarine can remain submerged for a substantial amount of time.

Inside a submarine, conditions tend to be very cramped. Whether scientific or military, a submarine is packed with gear pertaining to its use, and every available surface is utilized to maximum capacity. The staff of a submarine can be in the hundreds on larger submarines, which also requires extensive support services such as an infirmary, a mess hall, and accommodations. The power plant of a submarine must also be used to generate oxygen and keep the air in the submarine clean so that the interior is a healthy living environment.

Many people are familiar with the periscope, a device used to look above the water while a submarine is submerged. A periscope consists of a set of mirrors mounted inside a long tube. When the periscope is extended, a sailor puts his or her eye to the viewer and is able to see what is going on above the water. A periscope can be used to look for threats, check on weather conditions, or make other quick general assessments. Some submarines also have a snorkel system to intake fresh oxygen.