Neutral buoyancy diver maintains neutral buoyancy by continuous correction, usually by controlled breathing, as neutral buoyancy is an unstable condition for a compressible object in a liquid. The mathematician Archimedes discovered much of how buoyancy works more than 2000 years ago. In his research, Archimedes discovered that an object is buoyed up by a force equal to the weight of the water displaced by the object. In other words, an inflatable boat that displaces 100 pounds (45 kilograms) of water is buoyed up by that same weight of support. An object that floats in the water is known as being "positively" buoyant.
Neutral buoyancy suited astronaut trainees interact with mock-up space hardware, with the assistance of scuba divers. At the University of Maryland's Space Systems Laboratory, a neutral buoyancy tank is similarly used to evaluate the performance of prototype space robots. When neutral buoyancy is taking place, it appears as though the object/substance is floating in the middle of the fluid, or somewhere in between the bottom and the surface. A fish's swim bladder controls buoyancy by adjusting the amount of gas in the swim bladder, allowing it to achieve neutral buoyancy at different depths. When a fish's overall density becomes higher or lower