A Navy-funded thermal engine bobbing off the coast of Hawaii is accomplishing a rare feat -- it produces more energy than it consumes. Though it's not quite a perpetual motion machine, it could provide scientists or the Navy with a perpetual presence on the seas. The engine is attached to an unmanned underwater vessel, called SOLO-TREC, and uses the energy of the ocean to derive a practically limitless energy supply. SOLO-TREC is outfitted with a series of tubes full of waxy phase-change materials. As the float encounters warm temperatures near the ocean's surface, the materials expand; when it dives and the waters grow cooler, the materials contract. The expansion and contraction pressurizes oil, which drives a hydraulic motor. The motor generates electricity and recharges the batteries, which power a pump. The pump can change the float's buoyancy, allowing it to move up and down the water column. In theory what you have now is unlimited endurance for something that has this type of engine," said Thomas Swean Jr., team leader for ocean engineering and marine systems at the Office of Naval Research, which funded the project. "Other things can break, but as far as the energy source, it will only stop working if the ocean ran out of energy, which is unlikely to happen."