A payload release mechanism is provided, which operates utilizing a pressure differential between the mechanism and the surrounding medium. A piston is movably seated within a chamber of the release mechanism. The piston serves to seal the chamber from the surrounding medium, though one end of the piston is in communication with the surrounding medium. The pressure differential between the end of the piston in communication with the surrounding medium and the medium within the chamber causes the piston to move. The piston movement is used to trigger the release of the payload. The amount of movement which triggers the release can be calibrated to correspond to a predetermined pressure differential.