Advanced technology played role in construction of undersea tunnel

China's first undersea tunnel for a subway employed advanced technologies and machinery to combat complex geological conditions.

The cross-sea tunnel - an 8.1-kilometer segment, 3.49 km of which is underwater - spans Jiaozhou Bay in Qingdao, Shandong province, and links the coastal city's downtown area in the east with the Huangdao district in the west.

It is the country's deepest undersea tunnel, with its deepest section extending 88 meters below sea level. It bears water pressure stress of at least 8.8 kilograms per square centimeter. Other undersea tunnels have been built for vehicles, but this is the first for a subway.