Why is the U.S. drone seized by China so important?

The capture of an underwater drone belonging to the U.S. Navy by a Chinese military vessel last week cast a spotlight on the military’s growing interest in using such drones.

U.S. officials said the seized drone — an unarmed torpedo-looking device known as an autonomous underwater glider — was surveying the ocean floor in the South China Sea and collecting scientific data, such as information on water salinity, when China seized it Thursday. Reports say the captured drone was not one of the Navy’s most sophisticated unmanned underwater vehicles.

On Friday, the Pentagon demanded the return of the drone and the U.S. government issued a formal diplomatic complaint.

A day later, China agreed it would return the drone “in an appropriate manner.”

U.S. President-elect Donald Trump said on Saturday that China had stolen the drone in an “unprecedented act.” Later, he repeated the accusation in a follow-up Twitter message in which he said that China should keep the drone.

China seized the drone to protect shipping safety in the region, the country’s foreign ministry spokeswoman Hua Chunying told reporters in Beijing on Monday. The government didn’t like the word “steal” to describe its actions, she said, without mentioning Trump’s name.

China’s military is still in talks with U.S. counterparts over returning the drone, she said.

General military uses for these kinds of drones include stealthily gathering intelligence on opponents, detecting and neutralizing mines, hunting submarines and charting the ocean floor.

But it’s not just the military that’s increasingly interested in using sea drones. Oil and gas companies want them to inspect rigs, and scientific researchers have also looked to drones to collect oceanographic data.

Recent advances in autonomy, data transmission and miniaturizing computing power coincide with the military’s increased interest in finding ways to integrate unmanned systems into its war planning.