The U.S. Navy has approved the use of buoyancy gliders by all of its destroyers. These unmanned underwater vehicles use wave energy to propel themselves at sea. The Navy probably will use them to locate enemy submarines.
Buoyancy gliders were originally developed by the scientific community to provide low-cost, autonomous drones capable of spending long periods of time at sea. The drones look like cruise missiles, with blunt noses, a vertical stabilizer tail fin, and two small winglets.
Once released into the sea, the drone moves by taking water into its nose, causing it to sink and move forward. The glider is powered by a battery pack that can slide back and forth to change its center of gravity, changing its angle of descent. After reaching 150 to 180 meters, the drone expels the water, causing it to rise. The drone can rise all the way to the surface, whereupon it can check in with its operators, uploading data and downloading new instructions.
The obvious military use here e is to locate enemy submarines. The buoyancy glider has no engine, so it's virtually undetectable. A glider equipped with passive sonar arrays could take up station in an area where enemy submarines are known to transit and quietly listen. Once it detects a submarine, it could rise to the surface and broadcast an alert, passing on the submarine's speed and heading—and maybe even type—to friendly forces.