Ballast

Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability.

A compartment within a boat, ship, submarine, or other floating structure that holds water is called a ballast tank.

The basic concept behind the ballast tank can be seen in many forms of aquatic life, such as the blowfish or members of the argonaut group of octopus.

[citation needed] During World War 2 ships returning from Great Britain to the United States used rubble as ballast.

For example, in gliding it may be used to increase speed and/or adjust the aircraft's center of gravity, or in a balloon as a buoyancy compensator.