The 816 underground nuclear plant in Chongqing's Fuling district, named by some media as "the world's largest man-made cave," opened for the first time in more than 40 years to visitors, the Chengdu Daily reported Monday.

Visitors walk through a tunnel in the 816 underground nuclear plant in Chongqing's Fuling district, April 24, 2010. [Xinhua]

The plant, totally hidden in Fuling's Jinzishan mountain, saw its construction work start in 1967, but in 1984, when it was almost completed, the country called it off due to a favorable international environment for China. The plant was declassified in April 2002.

It is more than 20 kilometers long and has a total building area of 104,000 square meters. It also has 18 large main caves and more than 130 roads, branch caves, tunnels and vertical shafts. More than 60,000 people have worked for the plant during its 17-year-long construction.

There are roads for cars, wind tunnels, drainage channels and warehouses for storing weapons and food. In order to prepare for potential wars, the cave can withstand 8.0-magnitude earthquakes and atomic and hydrogen bombs, said a tour guide.

Now only the nuclear reaction hall, which is the largest cave in the plant with a height of 79.6 meters but accounts for only one-tenth of the whole cave, is open to the public, said Hu Lindan, a person in charge of the plant's maintenance.

Although the cave is open to visitors, there are strict limitations. There are soldiers standing guard at the entrance, and visitors are allowed in only when they have required certificates.