Buoyancy is the term which describes that a body tends to float or rise when submerged in a fluid. Buoyancy is an upward force exerted by a fluid--in this case, water on an object, which opposes the force of the weight of this object. Therefore, the upward force of buoyancy is equal to the magnitude of the weight of the liquid that the object displaces. In plain terms, as the depth of water increases, the pressure increases also. The pressure beneath someone who is swimming is greater than the pressure above and this is why someone can float. This principle was discovered by Archimedes, a Greek Mathematician.