Two of China's deep-sea submersibles have descended 10,000 meters below the surface of the Pacific Ocean to complete experiments in the Mariana Trench. The deep-sea craft are the second generation of the "Caihongyu" ("Rainbow Fish") robots. In this photo taken on Monday, December 10, 2018, researchers are seen preparing to land one of the submersibles on the seabed to collect seawater samples. [Photo: Xinhua]

Two of China's deep-sea submersibles have descended 10,000 meters below the surface of the Pacific Ocean to complete experiments in the Mariana Trench. The deep-sea craft are the second generation of the "Caihongyu" ("Rainbow Fish") robots. In this photo taken on Monday, December 10, 2018, researchers prepare one of the submersibles to take photos and collect biological samples. [Photo: Xinhua]

Two of China's deep-sea submersibles have descended 10,000 meters below the surface of the Pacific Ocean to complete experiments in the Mariana Trench. The deep-sea craft are the second generation of the "Caihongyu" ("Rainbow Fish") robots. In this photo taken on Tuesday, December 11, 2018 researchers recover equipment from the seabed. [Photo: Xinhua]

Two of China's deep-sea submersibles have descended 10,000 meters below the surface of the Pacific Ocean to complete experiments in the Mariana Trench. The deep-sea craft are the second generation of the "Caihongyu" ("Rainbow Fish") robots. In this photo taken on Tuesday, December 11, 2018 researchers recover equipment from the seabed. [Photo: Xinhua]