Seventy percent of the Earth is covered by water, but there is so little humans know about the vast oceans. To explore the unknowns, scientists have started descending into a part of the ocean known as the Hadal Zone – a name derived from Greek God of the Death – Hades. It's 6,000 to 11,000 meters underwater.

The Challenger Deep of the Mariana Trench in the Pacific is considered the deepest part of the ocean humans know – the very bottom. Japanese remotely operated underwater vehicle Kaiko reached the bottom of the trench in 1995, but it was lost during a typhoon in 2003. Hollywood Director James Cameron took a solo dive to the bottom back in 2012. However, some scientists think there is no actual submersible, manned or unmanned, which can dive to the bottom of the Challenger Deep for scientific works.

Professor Cui Weicheng is leading a project called the Rainbow Fish, aiming to send a manned submersible that can carry three scientists down to the Challenger Deep of the Mariana Trench.

“For the shallow parts of oceans, we've done research. But for the deepest part, there is a blank; no research at all. In order to finish that task for humans to utilize oceans scientifically, we need to develop that kind of system,” said Cui.

The professor believes that humans need to go down, see the trench, and study it before using marine resources responsibly. As one of the senior designers for the China-made Jiaolong submersible – still known as the operational sub with the deepest dive, the Rainbow Fish project was jump-started. An unmanned sub took a dive into the Mariana Trench in December 2016. While it did not reach the bottom, it showed that the project is feasible. Yet, from unmanned to manned, the cabin is the most difficult part.

"Particularly for these three windows, it's very challenging. Because this part is made from steel, and here, we made it with glass," said Cui.

Scaled down manned spheres were made, and put into a pressure chamber for testing. The chamber can simulate deep sea environment with pressure in fact 1.25 times of the pressure 11,000 meters underwater.

The team is still searching for the right steel to construct the manned balloon capsule so that it can endure pressure. They told CGTN that once the material failed, the cabin bursting inside the pressure chamber could shake the ground.

A concept model of the manned sub and its manned sphere have been made. And the team would make adjustments to allow the vehicle to depend faster if needed. The current estimate is that the sub would take about two and a half hours down to the deepest – allowing scientist to work at the bottom for five hours on a ten-hour-shift. Nevertheless, Cui says priority number one is building an operational submarine that can take scientists to the deepest part of the ocean and quickly, not just setting records.

He says going there will satisfy human curiosity, and give us invaluable information about Earth's prosperity.