Last year, with materials and interviews accumulated over the years, Gao completed the book in three months, trying to "dive deep into three questions" — human ideas about the ocean, why China develops advanced deep-sea manned submersibles, and the key technical challenges.

"Basically, there are three technical questions: how to dive, how to ascend from the deep sea, and how to communicate with the outside world," Gao says.

Take the giant titanium spherical cabin for example. Ideally, it should be lightweight yet withstand the high pressure trying to crush it in the abyss, while being big enough to carry three pilots and allow them to conduct their activities.

As the vehicles dive into deeper water, the surface of the cabin will take increasing pressure, and in the Mariana Trench, the pressure is 1,100 times of that on the ground.

When Chinese scientists started developing the country's first deepsea manned submersible Jiaolong in 2003, they did not even know what shape the vehicle should look like. The chief designer Xu Qi'nan, born in 1936, once worked on a submarine, and based on this experience, he estimated that the submersible should also be streamlined, says Gao.

At the start of the century, China was short of talent specializing in submersibles. Xu, a retired engineer returning from the United States, decided to nurture a research team and entrusted 12 young people with major tasks.

One of them was 24-year-old Ye Cong. At university, Ye studied ship engineering and before joining the team, his only experience with related research was his participation in the development of a submarine rescue ship that could dive to a depth of 600 meters.