Unlike their airborne cousins, Navy gliders are not powered by aviation fuel. Instead, they draw energy from the ocean’s thermocline, a pair of layers of warm water near the surface and chillier water below.

The glider changes its density, relative to the outside water, causing the 5-foot (1.5m)-long torpedo-like vehicle to either rise or sink—a process called hydraulic buoyancy. Its stubby wings translate some of that up-and-down motion into a forward speed of about a mile (1.6 km) an hour in a sawtooth pattern. As it regularly approaches the surface, an air bladder in the tail inflates to stick an antenna out of the water so it can transmit what it has learned to whatever Captain Nemo dispatched it to the depths.

Much of the work such gliders do is oceanographic in nature, collecting data about the water’s temperature, salinity, clarity, currents and eddies. Such information is critical for calibrating sonar to ensure it provides the most accurate underwater picture possible. But there are additional efforts underway to convert such data into militarily-handy information.