Buoyancy control device (BCD) or buoyancy compensation device is a body harness with an integrated air bladder that can be inflated and deflated, either manually or by the power inflator. The reason why a diver wears a BCD is to compensate for the loss of buoyancy as he or she descends, due to the effects of increased pressure on the exposure protection suit. Without the BCD, most divers would be negatively buoyant and sink. By adding air, the BCD allows you to be neutral at any depth and positive on the surface. Remember Archimedes’ principle. If we make ourselves bigger by inflating the BCD, we will displace more weight of water, making ourselves more buoyant.