SANYA -- Chinese scientists have completed manned deep-dive expeditions in eight major hadal trenches across the globe, achieving multiple world records related to manned deep-sea missions and discoveries.

This was revealed at the second International Conference on the Biology, Geology and Environment of the Hadal Trenches, which opened Thursday in Sanya of the southern island province of Hainan.

The conference has gathered experts from countries such as Denmark, France, Portugal, New Zealand, Chile, the United States, India and Indonesia.

The world has 37 hadal trenches, which are ocean zones exceeding 6,000 meters in depth. The hadal trench environment is characterized by high pressure, low temperatures, permanent darkness, active tectonics, dense earthquakes and strange life forms. These trenches are a frontier in the earth sciences, particularly in the marine sciences.

China's scientific research on hadal zones involves the use of the Fendouzhe submersible and has made remarkable progress in recent years. To date, China has completed expeditions to the Mariana Trench, the Yap Trench, the Kermadec Trench, the Diamantina Trench, the Wallaby-Zenith Fracture Zone, the Java Trench, the Kuril-Kamchatka Trench and the Aleutian Trench.

The three-day conference is being hosted by the Institute of Deep-sea Science and Engineering under the Chinese Academy of Sciences.