A submarine is a ship capable of operating below the surface of the water. The first submarine is reported to have been first built in 1623 http://www.submarine-history.com/NOVAone.htm. It operated 15 feet under the surface of the Thames River, and was little more than a sealed up rowboat. Modern submarines are capable of operating below the surface for months at a time and can safely achieve depths over 400m or 2000 feethttp://www.fas.org/man/dod-101/sys/ship/deep.htm.
Submarines have many uses, although military submarines have achieved the most notoriety, due primarily to the German development of the U-Boat http://www.uboat.net/ and the modern nuclear submarine. Submarines are also used extensively for researching the oceans depths, for salvage and repairs, and occasionally for tourismhttp://www.atlantisadventures.com/hawaii/kona/kona-activities.cfm/<ref>.
Submarines are able to dive beneath the surface in two ways. The first is by changing the angle of the boat so that the boats speed forces the submarine down. The second is by using Ballast Tanks to change the buoyancy of the boat. <ref>http://www.heiszwolf.com/subs/tech/tech01.html. Ballast tanks work by taking on water to make the boat heavier, allowing it to sink under the water.
The Soviet Typhoon class submarine is the largest submarine ever builthttp://www.battle-fleet.com/pw/his/Largest%20Submarine%20Typhoon.htm. It displaces an incredible 48,000 tonnes, and was designed to carry 20 intercontinental ballistic nuclear missiles (ICBM).
Specially designed deep-sea submarines have reached depths of over 10,000m (6.8 miles)http://news.bbc.co.uk/2/hi/science/nature/8080324.stm. To get that deep, the submarine's hull must withstand a pressure over 1,100 times the pressure felt at the surface.
