Finding a natural, renewable, eco-friendly way to power machines is the holy grail of propulsion research. A team comprised of university researchers, NASA, and the US Navy recently unveiled an unmanned underwater vehicle that is powered entirely by geothermal energy. The vehicle, called the Sounding Oceanographic Langranian Observer Thermal Recharging (SOLO-TREC) uses the ocean’s changing temperatures to generate power. Thanks to phase change materials which melt at higher temperatures and solidify at lower ones, pressure changes in the engine trigger a hydraulic motor that recharges the vehicle’s batteries.

SOLO-TREC is an important breakthrough for marine biology, ocean exploration, and climate study. A vehicle that can continuously explore its surroundings without the constraints of an external (as opposed to environmental) fuel source has almost limitless applications. Rather than having to surface for refueling, the vehicle is refueled constantly by the environment in which it operates. The plucky little robot recently completed three months of ocean endurance testing off the coast of Hawaii, and its creators are planning to keep the study going for many months or years. And if you’d like to follow along as the bot traverses the ocean, take a look at NASA’s online map detailing its travels.