Archaeologists say they've begun examining the wreck of an ancient Chinese merchant ship currently preserved in a giant tank of water. The wreck dubbed Nanhai No. 1, dating to the Southern Song Dynasty in the 13th century, was raised from the bed of the South China Sea at the end of 2007 along with much of its cargo. Since then the 100-foot-long ship has been preserved in a special water tank at the Marine Silk Road Museum in Yangjiang, a city in south China's Guangdong Province. Museum officials said the work of examining the wreck is expected to last three to four years, China's state-run Xinhua news agency reported Friday. Archaeologists said they believe the wreck may contain 60,000 to 80,000 antique items including porcelain, precious metal objects and coins. Two trial excavations in 2009 and 2011 yielded more than 6,000 pieces of porcelain, copper and other treasure, they said.