This past week, a submarine rescue vessel from the Chinese People's Liberation Army Navy (PLAN) came up along the USNS Bowditch, a US Navy civilian oceanographic vessel—and sent out a boat to snatch one of two uncrewed ocean "gliders" being recovered by the Americans. The snatch-and-grab triggered a week-long diplomatic standoff accompanied by tweets from the President-elect:

China steals United States Navy research drone in international waters - rips it out of water and takes it to China in unpresidented act. — Donald J. Trump (@realDonaldTrump) December 17, 2016

We should tell China that we don't want the drone they stole back.- let them keep it! — Donald J. Trump (@realDonaldTrump) December 18, 2016

China gave the glider back today. A Chinese Ministry of Foreign Affairs spokesperson claimed that his country's sailors saw the glider as a potential hazard and were acting responsibly when they grabbed it. "I want to say we strongly dislike the term 'steal' as it's entirely inaccurate," the spokesperson said. Earlier, Chinese officials claimed that the gliders were likely being used to spy on China's operations in the South China Sea or perhaps locate routes traveled by PLAN's submarine fleet.

The gliders are long-endurance, slow-moving submarine drones that are used for oceanographic data collection. They can be remotely controlled on runs lasting as long as 30 days, and they use shifts in their own buoyancy to propel them.

The data the gliders collect is itself relatively benign—things like water salinity, temperature, and pressure. The information they provide to oceanographic survey ships like the Bowditch helps scientists understand the thermodynamics of the areas that the gliders chart using sonar. Thermal layers in ocean water can scatter sonar signals and create distortions in the returned "pings" of active sonars.

According to the Navy's Military Sealift Command, the Bowditch's mission is to survey swaths of ocean, using "multi-beam, wide-angle, precision hydrographic sonar systems to collect water depth measurements and other related data." In other words, the Bowditch maps the sea floor by towing sonar arrays behind it. The multi-beam sonar is used for contour mapping of the ocean floor, particularly in areas where currents and storms might shift the position of potential hazards on the bottom (as it did following a 2013 typhoon). The glider's data helps ensure the accuracy of these maps—maps that could be used, say, by a US Navy submarine traversing the South China Sea. It can also be used to improve sonar accuracy for other reasons—for example, in tracking submarines.

That mission has brought the Bowditch to the (rather aggressive) attention of the Chinese on several occasions in the past. On March 24, 2001, the Bowditch was threatened in the Yellow Sea by a PLAN frigate, and it was forced to leave the area and return with an armed escort. Chinese patrol boats and aircraft again harassed the Bowditch in 2002 in the Yellow Sea, again forcing the ship to leave. And in 2004, a Chinese fishing boat rammed the Bowditch.