Submarines are ships able to move under the water in any direction. They are often called submersibles as well.

Most of the submarines are used for military purposes. They are often used for reconnaissance and sometimes for rescue missions. In war periods, they are also used during naval combats to fire torpedos that can seriously harm the opposite boats. Some submarines can carry nuclear weapons and represent a nuclear deterrent.

Even if it is quite scarce, some submarines are not used for military purposes. Some of them are only used so as to perform oceanic research and explore the bottom of the sea. Oil industries also use submarines in order to complete several tasks necessary for the extraction of the oil.

To easily move under the seas, submarines have to be built according to two essential physics principles about immersed objects : the principles of Archimede and Pascal. In this way, it is essential to define with accuracy the weight of the submarine for it to be equal to the vertical push of the water, and use the suitable materials to make a hull thick and solid enough so as to resist to the barometric pressure.