This file photo shows a submersible under construction at an undisclosed location. [China Daily]

A domestic submersible has reached a depth of 3,759 meters, making China the fifth country in the world to acquire deep-diving technology surpassing the 3,500-meter mark, authorities said on Thursday.

The Chinese submersible Jiaolong, which is 8.2 meters long and weighs nearly 22 tons, was designed to reach a depth of 7,000 meters and operate in most of the world's oceans, officials said.

Jiaolong is considered to be the world's only deep-sea vessel that can theoretically reach those depths. Japan's Shinkai 6500 has a depth capability of 6,500 meters.

The other three countries with deep-diving technology are the US, France and Russia.

A submersible differs from a submarine as it typically depends on another vessel or facility for support.

Jiaolong, with a crew of three, dived 17 times from May 31 to July 18 in the South China Sea, going below 3,000 meters four times. The deepest it reached was 3,759 meters. It operated at a peak duration of nine hours and three minutes and sent back videos and photos of the seabed.

In an experiment when it landed on the seafloor, it placed the Chinese flag and a sign depicting the legendary dragon's palace onto the seabed with a robotic arm.

"The successful diving trials of Jiaolong marked a milestone in our country's deepwater equipment and technology development," said Wang Weizhong, vice-minister of science and technology.

China started to develop the submersible in 2002. Work on the deep-sea vessel and its mother ship were completed after six years and involved about 100 institutions and companies nationwide.

"During the design and building stage, we overcame many technical difficulties such as pressure resistance, auto-control systems and battery capacity," Wang Fei, deputy chief of the State Oceanic Administration, said while introducing the submersible.

Jiaolong started its diving trials in August last year by going through depth stages.

"From last year's 50 meters, 300 meters and 1,000 meters to this year's depth of more than 3,700 meters, it is a great achievement. Such a depth means that Chinese scientists are able to conduct research in the deep. It also marks China becoming one of the few countries that possess manned deep-diving technology," said Liu Feng, chief director of the diving trials.

Lack of experience was the biggest difficulty faced in the trials, Liu said, adding that severe weather such as typhoons and storms also hampered efforts.

"We did pressure tests that simulated the environment at 7,000 meters underwater. That depth is our target. We will keep on trying deeper depths and finally reach the 7,000-meter goal," Liu said.

Ye Cong, one of the hydronauts on board Jiaolong, said he could sit in the vessel while the other two crew members could only stoop in the cabin.

"The air pressure in the cabin is the same as it is on the surface. I felt excited and nervous every time I went deep underwater," Ye said.

Ye said the team had just successfully completed the diving trials and it still had a long way to go before the submersible was applied to scientific research.

China's submersible development is aimed at scientific research to help with the peaceful exploration and utilization of natural resources, officials said.

Jiaolong's main missions include physical, chemical and biological research, as well as exploration and deep-sea salvage, officials said.

(China Daily August 27, 2010)