China also maintains underground nuclear weapons facilities, similar to those found in Iran, according to defence analysts and intelligence agencies. The most famous is “project 186”, which was closed down after China signed the Non-Proliferation Treaty (NPT) and is now a tourist attraction, but other bases continue to operate.

The project was born in the 1960s, during the so-called Sino-Soviet split. Tensions with the Soviet Union were high and Beijing wanted to develop nuclear weapons in protected sites around the country. Project 816 in the Sichuan province was one of the biggest and most sophisticated.

Now a tourist attraction in Baitao Town in Chongqing's Fuling District about 150 km east of the city, that town was completely wiped off all maps after the base was established and all the residents in the area were evacuated. Some 60,000 soldiers were drafted for the construction work that lasted 18 years to build what was simply referred to as the "State-Owned Jianxin Chemical Machinery factory".

“It is probably one of the most insane secret military facilities ever built that we know about,” says China expert and bne IntelliNews columnist Arnaud Bertrand, who has visited the site.

The Iranian nuclear enrichment projects are based on refining uranium that Iran has in abundance but China’s 186 project was based on using plutonium-239. The highly radioactive isotope does not exist naturally and is made in nuclear reactors.

Pu-239 is fissile, meaning it can sustain a chain reaction simply by firing slow neutrons into it at normal temperatures, making it valuable as both a nuclear fuel in reactors and as a weapon. In nuclear reactors the Pu-239 slowly absorbs neutrons to become Pu-240 that is not fissile and becomes spent fuel that must be removed. Because of its low rate of spontaneous fission, Pu-239 is ideal for nuclear weapons, as detonations can be precisely timed.

“All in all, the scale of the project is just unbelievable: it covers 104,000 square metres of floor space (the equivalent of 26 football fields). It took 60,000 PLA soldiers 18 years to build,” Bertrand said in a blog about the project. “The reactor itself (yes they built a full-blown nuclear reactor in a cave) was located in the world's largest artificial cave, which they dug 30 metres under the bottom of a river, using the water of the river to cool it down.”

Project 816 was built so it could survive several nuclear strikes, so if Iran’s Fordow nuclear facility, that was targeted by US banker busting bombs at the weekend, “is anything even remotely close to it, hard to imagine that Trump's bunker-busting bombs killed it,” Bertrand said.

In the 1960s China had jointly developed its nuclear programme with the Soviet Union but it urgently needed to have its own independent capabilities, developed in secure facilities. The plan was to build a complete nuclear production site in man-made tunnels and a huge cave, including an underground nuclear reactor capable of producing the enriched Plutonium-239 needed for the bombs.

“That's why the site is right next to the Wujiang river: the artificial cave with the nuclear reactor, which is the world's largest artificial cave (a crazy 79.6m high!), sits 30m under the river: in operations, a third of the river's water would be diverted to cool down the reactor,” says Bertrand. “

The only visible sign from the outside that there might be something there was relatively humble tunnel entrances and a non-descript chimney up in the hills.”

The 60,000 soldiers who worked on the project were sworn to lifetime secrecy, even to their families, and most didn't even know what they were working on: as far as they were concerned, they were just digging tunnels.

“That's not all, the place was built so it could withstand bombing by the equivalent of several atomic bombs, and a magnitude 8 earthquake,” says Bertrand. “Simply put, I think it's one of the largest-scale military infrastructure projects of the 20th century that we know of.”

The whole project was eventually cancelled in 1984 after China's commitment to nuclear non-proliferation and the changed international context, including the reducing of tensions with the Soviet Union and with the US. But its existence was only revealed to the public in 2002 and was opened to the public as a tourist attraction in 2010.

While project 186 is the most famous, Bertrand says there are actually several other similar projects where different designs were being tested such as "project 827" – a similar, but much less famous, project that was based on a heavy water moderated type nuclear reactor, not the light water cooled graphite moderated type of reactor used in the 186 project.