The structure of the single vacancy in silicon. one of the most common point defects and an important mediator of atomic diffusion, is examined through extensive first principles calculations. We find a hitherto unexpected result, namely that there exist two distinct distortions associated with the vacancy with essentially identical formation energies at zero pressure. The two distortions are distinguished by their different relaxations, volumes of formation, and :formation enthalpies. We discuss how, at finite pressure, one of the two distortions should become dominant, and suggest experimental tests of this effect.811020882091