Tansuo-2, the second mother ship of China's new deep-sea manned submersible Fendouzhe (Striver), berths at a port in Sanya, south China's Hainan Province, Dec. 21, 2020. China's scientific research ship Tansuo-2 returned to port in the city of Sanya, south China's Hainan Province on Monday, after completing its ocean expedition. (Xinhua/Zhao Yingquan)