A Chinese research vessel, carrying an underwater robot and an unmanned submersible, on Tuesday completed a month-long deep sea exploration in the western Pacific Ocean.

The vessel docked in Sannya city, Hainan province, reports Xinhua news agency.

It set off from Shenzhen on August 7, travelled the South China Sea, and sailed to the Yap Trench for ecological and marine organism surveys.

Xu Kuidong, chief scientist on board the ship, said the researchers sent the unmanned submersible 15 times to Caroline Seamount, where no exploration had been made before, to collect 400 marine samples from 170 species.

They also found coral gardens and sponge forests in the area.

On the expedition, a piece of coral found 1,246 metres deep might be older than 4,200 years, the age of the oldest coral ever found. Researchers will do lab research to check its age, Xu said.

The researchers believe that Caroline Seamount was once an island. It subsides by at least 1,500 metres during plate movements.