Archimedes’ Principle

An object lighter than the area of fluid it occupies will float. Therefore, an object heavier than the area it displaces will sink. Archimedes’ principle explains why a 1000 ft. ship weighing over 200 million pounds floats. Yes, it explains why things float and sink in water. But it also explains why a balloon can float on air. Buoyancy is always an upward force and the object’s weight is the downward force. 

In SCUBA diving there are two pieces of gear that affect buoyancy, the Buoyancy Control Device (BCD) and your weight system. The addition of weight overcomes positive buoyancy allowing you to sink. Adding air to a BCD will increase total volume/size, displace more water, and increase upward buoyant force. You adjust buoyancy by adding or removing air from the BCD.

Buoyancy enables sunken objects to be raised from the depths. Never attempt to use a lifting device without adequate training and experience. A diver should never pick up a heavy object underwater and attempt to use their BCD as a lifting device. If the object is released, an uncontrolled ascent is usually the result.