China's deep-sea submersible mother ship, Tansuo-1, returns to Sanya in south China's Hainan Province, March 23, 2017, after completing its 68-day voyage for the Mariana Trench expedition. The ship serves as the mother ship for a manned submersible that can reach a depth of 4,500 meters. (Xinhua/Yuan Yongdong)
