The once-secret structure has caves, tunnels, and roads to explore.

By Talia Avakian
October 03, 2016
Credit: Xinhua News Agency/Getty Images

Tourists visiting Chongqing in China can now explore an underground nuclear base, dubbed the world's largest manmade cave at a size of 1.1-million square feet (or about 20 football fields).

The abandoned 816 Nuclear Military Plant is open to foreign visitors for the first time after undergoing renovations that include new sound and lighting effects throughout its corridors, a new nuclear science center, and various sections explaining the history behind the base, according to China Daily.

Inside, visitors can explore more than 12 miles of caves, with more than 130 different roads, tunnels, and shafts to wander through. The largest cave—the nuclear reactor hall—scales a height of more than 262 feet and a width of 82 feet.

Credit: Xinhua News Agency/Getty Images

Construction on the architectural feat, nestled in the mountains of the Fuling District, began in 1966. More than 100 workers died during construction, according to China Daily.

The base was originally opened in 2010 for Chinese visitors, and will now include portions previously closed off to tours.

Talia Avakian is a digital reporter at Travel + Leisure. Follow her on Twitter at @TaliaAvak.