A device for generating substantially vertically-reciprocating motion uses a plurality of chambers in substantial vertical alignment. Each chamber has an air pocket and a buoyant member which floats at the air/water interface, and such air pockets expand and contract upon receipt of additional amounts of air from below and release of some of the air, usually to a chamber above. As the air pockets expand and contract, the interfaces are raised and lowered, and the buoyant elements are raised and lowered with the interfaces. A method uses such device to generate such reciprocating motion, which optionally may be used to drive electric generators.