Similar to boats and ships, submarines use the concept of upthrust to sink or float. Upthrust is created by water that is pushed out of the way (displaced) by an object. Submarines change their weights depending on whether they wish to dive under the water or rise to the surface.
If a submarine is about to dive, its weight is increased by pumping water into large tanks. When the weight of the submarine is greater than the upthrust it creates by displacing water, it begins to sink. The more water that is pumped into the tanks, the heavier the submarine, and the faster it sinks.
When the submarine has reached the required depth, some of the water is pumped out of these tanks to decrease its weight. When the upthrust and weight are equal, the submarine will neither rise nor sink. It will remain at that depth. If the submarine is to surface, more water is pumped from the tanks so that the upthrust is now greater than the weight.
Source: Floating and Sinking. (2010). In The Hutchinson Unabridged Encyclopedia with Atlas and Weather guide. Retrieved from http://www.credoreference.com/entry/heliconhe/floating_and_sinking
(Last accessed: 26 March 2011)
Originally answered by Rosjihanah Mon
Associate Librarian, Children’s Services