In one of the stranger escalations of tensions between the US and the Chinese, a Chinese naval vessel seized one of USNS Bowditch drone gliders. USNS Bowditch is an oceanographic survey vessel which operates with a mix of civilian and military crew members. The Chinese ship, a Dalang-III class submarine rescue vessel, put a small boat in the water which intercepted the drone and took it back to the Chinese ship, which then left the area. The incident took place about 50 nautical miles northwest of Subic Bay, in international waters in the South China Sea. The US has issued a formal protest and demanded the return of the glider.
The drone glider is an autonomous underwater vehicle, (AUV), typically around 6 feet long, which uses changes in buoyancy and trim to propel itself through the water. Press reports have described the drone as either a Slocum glider or a Seaglider, which are variants on the same glider type. These sorts of drones are widely used by oceanographic institutions and commercial operations. See also our post from 2013, Underwater Gliders Gather Data to Help Predict Hurricanes.
The video below demonstrates the operation of a glider drone.