"BCD is an abbreviation for buoyancy compensating device. These useful and necessary pieces of equipment enable a diver to compensate for negative buoyancy by adding air from the tank to the BCD while underwater. They're also necessary for sustaining positive buoyancy on the surface. Without them, no diver could stay submerged for more than a few minutes at a time." Buoyancy compensating devices are used by scuba divers to maintain neutral buoyancy during various stages of the dive. The two most common types of BCDs are full-face masks and partial-mask assemblies. Full-face masks provide complete protection from water pressure with the exception of the wearer's nose and mouth. Partial-mask assemblies offer limited protection against water pressure but do allow for some breathable air to reach the wearer's face. As its name suggests, a BCD can only compensate for your own weight. It cannot overcome heavy weights such as those imposed by large amounts of rock or coral. In such cases you would need another form of propulsion to explore these sites. The term "buoyancy compensator" may cause some confusion since this is not the only type of equipment used by scuba divers. Other essential pieces of diving equipment include fins for walking on the surface of the water (not included in a BCD), a light source such as a flashlight or headlamp, and a mouthpiece for breathing underwater.