Abstract: An apparatus characterized by a first conduit communicating with a fluid-filled first enclosure and having an open end adapted to be immersed in a wave-forming body of fluid for conducting pressure-induced flow toward and away from the first enclosure, a variable volume enclosure disposed within the first enclosure having its lower wall connected to a structure of variable buoyancy, adapted to expand and contract in conformity with the principles of a "Cartesian diver", a second conduit connected for providing a flow of fluid between the first and the variable volume enclosures, and a motor connected in the second conduit and adapted to be driven by the fluid flow therein.