Wang has worked as a diver with China National Offshore Oil Corporation for nine years.

Born in Hubei province and raised near the Yangtze River, Chu believes his experience serving in the navy as a diver for eight years taught him how to deal with tense situations and reduce risk.

Chu dives to depths less than 60 meters underwater, which is called air diving, similar to Scuba diving.

In the Bohai Sea, diving is not for pleasure - there is no beautiful coral or fish to view. The water is pitch black, and divers are usually working blind.

"Eyes are useless underwater and I must have everything in my head before diving," Chu says.

Running out of air is just one hazard of Chu's work environment. The ocean puts the body's physical limits to the test. There is extreme cold, around 4 C, and increased physical pressure, and a sudden change can be fatal.

Fast water flow, deserted fishing nets and ice floes at sea make the underwater work more challenging.

No one has more painful memories than Chu from China's sea ice threat from December 2009 to February 2010.

In one of his dives in the Bohai Sea over that period, Chu stayed in the icy water for more than two hours, he soon could not feel any part of his body.