World's largest underground nuclear plant in China's Chongqing

Published : 2025-12-18

Editor︰Annie Chan

Author︰Wen Zhushi

Deep beneath a small hill along the Wu River in China's Chongqing lies the world's largest underground nuclear plant—the 816 Nuclear Military Plant.

Once a top-secret project from the 1960s, it now stands as a tourist attraction. What stories lie hidden within this "underground city"?

Chongqing 816 Nuclear Plant: How many caves and chambers are there in total?

Project 816, fully named "The Underground Atomic Reactor and Chemical Reprocessing Plant of the Third Front Construction" (三線建設進洞的原子能反應堆及化學後處理工程), is located deep in the mountains of Fuling District, Chongqing City.

Its total area reaches 104,000 square metres, roughly equivalent to the size of 15 football pitches, and it is the world's largest man-made cave complex.

Chongqing 816 Nuclear Plant tunnel — After entering through the public entrance to the scenic area, one must pass through a 400-metre-long tunnel to reach the reactor hall in the plant area. (Image Source: Getty)

This underground nuclear plant has a total of 18 large chambers, and over 130 roads, guide tunnels, branch tunnels, tunnels, and vertical shafts, resembling a giant underground labyrinth.

Its engineering design, described as "buildings within caves, caves within buildings, and rivers within caves", won a collective award at the National Science Conference in 1978.

Chongqing 816 Nuclear Plant's nuclear reactor hall — The nuclear reactor hall of the 816 Nuclear Military Plant is located over 30 metres below the bed of the Wu River, with the main plant building reaching a height of 79.6 metres. (Image Source: VCG)

The protective layer covering the top of the entire plant is up to 200 metres thick, with the thickness over the core sections being more than 150 metres, which allows it to withstand the impact of a one-million-tonne hydrogen bomb airburst and a direct hit from a 1,000-pound bomb, as well as resist a magnitude-8 earthquake.

The base of the reactor at the Chongqing 816 Nuclear Plant — The base of the reactor at the 816 Nuclear Military Plant. (Image Source: Getty)

Since the plant was never put into operation and never stored any nuclear materials, there is no need to worry about radiation issues.

It was built in the 1960s and was China's second nuclear raw material industrial base at the time, primarily to provide raw materials for manufacturing atomic bombs.

The largest underground nuclear plant: Construction halted for peace

In the early days after the founding of the People's Republic of China (PRC) in 1949, why was such a vast sum of money spent on building a nuclear plant like this? This has to do with the historical context of the time.

Entire tour of the Chongqing 816 Nuclear Plant — Over 30% of the 816 Nuclear Military Plant is open for tours, and one tour takes approximately 3 hours. Visitors need to follow the guide's instructions, otherwise they risk getting lost in the giant underground labyrinth. (Image Source: Getty)

After the Second World War, whilst the nuclear-armed superpowers of the U.S. and the former Soviet Union struggled for hegemony, the Vietnam War erupted. With tensions simmering on the Indian border and relations with the Soviets souring, China was, in effect, under attack from front and rear.

Out of security concerns, the national leaders of China at the time decided to establish a strategic rear, which is the famous "Third Front Construction".

Central control room of the Chongqing 816 Nuclear Plant — The central control room of the 816 Nuclear Military Plant once housed China's most advanced computer mainframe at the time and was the "heart" of the entire project. (Image Source: Getty)

The coastal areas were the first line; the central regions were the second line; and the third line referred to the vast region east of the Wushaoling Mountains (烏鞘嶺) in Gansu Provine, west of the Beijing-Guangzhou Railway, south of Yanmen Pass (雁門關) in Shanxi Province, and north of Shaoguan (韶關) in Guangdong Province.

Instrument room of the 816 Nuclear Military Plant — Inside the instrument room of the 816 Nuclear Military Plant, over 1,034 instruments have been perfectly preserved, which were mainly used at the time for monitoring faults in the process pipes. (Image Source: Getty)

In the strategic rear area, a nuclear industrial base with greater concealment and security became an urgent necessity for China's military defence at the time, and so the "top secret" military Project 816 came into being.

Abandoned electrical switch at the Chongqing 816 Nuclear Plant — Abandoned electrical switches at the 816 Nuclear Military Plant. (Image Source: Getty)

However, the international situation was changing rapidly.

In 1984, peace and development became the global mainstream, and construction on the 816 Nuclear Military Plant, which was not yet completed, was officially halted.

Thanks to the 17-year efforts of tens of thousands of workers, 85% of the cavern's construction work and 60% of the installation work had been completed by then.

Declassified in 2002: Nuclear plant turned into tourist attraction

In 2002, China approved the declassification of Project 816, and this top-secret factory, abandoned for many years, was revealed to the world for the first time.

Entrance to the Chongqing 816 Nuclear Plant — The underground plant has a total of 19 openings of various sizes on the mountainside, fully equipped with personnel entrances and exits, vehicle access tunnels, ventilation shafts, drainage channels, warehouses, and more. The current entrance to the tourist attraction is one of them. (Image Source: Getty)

In 2010, parts of the cavern complex were opened to the public as a tourist attraction. It simulates and restores some functions of the nuclear project, allowing visitors to relive that special period of history. On its opening day, it attracted more than 6,000 tourists.