The Slocum Glider, an unmanned submarine from Teledyne Webb Research, Falmouth, Mass., takes a different approach to moving through the water. When it’s on or close to the surface, the Glider deflates an air bladder, making the 104-lb vessel heavier than the surrounding water. As the drone falls through the water, wings provide forward motion and steering. Upon reaching a targeted depth — it can dive as deep as 3,000 ft — the bladder inflates, and the Glider becomes lighter than water and rises to the surface. The Glider uses the downward part of the journey to make forward motion. This process is then repeated to move along a preprogrammed path.

This approach to locomotion uses little power, letting the Glider roam the waters for up to five years, or up to 930 miles, on a single set of alkaline batteries. During that time, onboard sensors collect data on the ocean or search for enemy subs or mines. Researchers and engineers are free to design any type of sensor packages for the drone (as long as it fits in the 7-liter payload bay.)

So far, sensors have included acoustic probes, hydrophones, optical attenuation detectors, and spectrophotometers. When the 5-ft-long Glider is near the surface, the tail section can be made buoyant so the rear-mounted fin antenna sits above the waves. It uses a modem to uplink collected data to a shore station or nearby ship. While it’s surfaced, it can also download further instruction and programming.