China returns US drone
China returned the US underwater drone taken by a Chinese naval vessel in the South China Sea last week after "friendly" talks between the two countries, China's Defence Ministry said on December 20, 2016. In Picture: Crew members aboard the VOS Raasay recover U.S. and British Royal Navy ocean gliders taking part in the Unmanned Warrior exercise off the northwest coast of Scotland on October 8, 2016.Reuters

The Chinese Navy has returned the Ocean Glider, an underwater research drone of the United States Navy, days after seizing it in international waters of the disputed South China Sea last week. Pentagon has maintained that the drone or unmanned underwater vehicle (UUV) was illegally seized by China.

China has also been accused of militarising the South China Sea and claiming the whole region as its Exclusive Economic Zone. The aggressive posturing means that China has picked up disputes with countries like Philippines, Indonesia and Vietnam.

But what is a UUV?

UUV has the ability to operate underwater without the need for human occupant. They are smaller in size.

There are two types of UUVs: remotely operated underwater vehicles (ROVs) and autonomous underwater vehicles (AUVs). ROVs are controlled remotely by a human operator, but the AUV is like a robot, is autonomous. It navigates using the onboard computers and sensors.

What do they do?

The drones are used for ocean science. These ocean gliders are equipped with wide variety of sensors that aid in monitoring temperature, salinity, currents and other ocean conditions.

There are some underwater drones that are used to track the movements of tectonic plates to improve early warning systems for tsunamis and earthquakes.

They also help in detecting threats of piracy.

Naval drones are used to check for mines dropped by enemy vessels.

What is the US Navy's UUV?

The US Navy's UUV was an AUV and a glider, which uses large moveable fins. It is a commercially-based system.

The US Navy's glider "is an unclassified system used to collect oceanographic data such as temperature, salinity, sound speed and water clarity."

What was the UUV doing?

The drone was deployed by US oceanographic survey vessel USNS Bowditch. Bowditch was in the process of recovering two ocean gliders.

The incident took place as the US Navy was retrieving them. The first glider was onboard and was about to get the other one too, this is when the Chinese ship slipped in and intercepted the second glider, CNN reported.

How much does the US spend on drone development?

According to analytics firm Govini, the US has spent about $605 million for UUV programs. It added that UUV research and development amounted to $138 million in 2016.

Will China remove intelligence data from the drone?

China had said that it would be retuning the drone "in an appropriate manner" to the US. China would return the drone, but without the information the drone had gathered, said state controlled Chinese website Global Times.