Abstract: Devices and methods are provided for generating a high pressure gas such as steam utilizing as a heat source the heat released by the change of state of a material, e.g. the heat of fusion released by solidification of a liquid metal. One such device generates steam in a continuous manner, while another such device generates steam in intermediate bursts. Also provided is a method for generating sonic waves underwater for seismic exploration. These sonic waves are generated from bursts of steam produced from the above-mentioned steam generating device which generates intermediate bursts of steam.