I guess it makes sense that the buoyant force of an object is equal to the weight of the displaced fluid, but why should this be the case? Why should that displaced liquid provide an upward force on the object? It seems apparent that as long as there is some part of the object above the surface, the result of the pressure on the submerged portion would net an upward force. But what about a completely submerged body? The pressure is now applied to every surface of the body (including the previously un-submerged surfaces), so why the net upward force? Is there some application of a conservation law that I am missing?