• Georgetown University student spent three years translating secret military documents and blogs
• Focus of research centred on 3,000 miles of underground tunnels dug by Second Artillery Corps
• Investigation launched after 2008 earthquake in Sichuan revealed existence of collapsed tunnels
• Study claims China could have as many as 3,000 nuclear warheads - far more then current estimates of between 80 and 400

China's nuclear arsenal may be many times larger than estimates suggest, a group of students have claimed after three years of painstaking researching through restricted documents.

Led by a former top Pentagon official, the students at Georgetown University, in Washington DC, have scrutinised satellite imagery, translated Chinese military documents and filtered through thousands of online files.

The focus of their extensive research has been the thousands of miles of underground tunnels dug by the Second Artillery Corps to hide China's missile arsenal.

Stockpiling: Missiles being test-fired at an undisclosed location. A group of university students have uncovered evidence that China may have many more nuclear weapons than previously thought

After the massive earthquake which struck the Sichuan province in 2008, news reports showed that thousands of radiation technicians were rushing to the region and images of collapsed hills led to speculation of the existence of an extensive tunnel network - which the Chinese later admitted did exist.

In those tunnels, the students have managed to draw up a picture of the sheer scale of China's nuclear capabilities - thought to be far greater than the 80 to 400 warheads the country was widely assumed to have.

The 363-page study has already sparked a congressional hearing and has been viewed by top Pentagon officials.

One Defense Department strategist told the Washington Post: 'It's not quite a bombshell, but those thoughts and estimates are being checked against what people think they know based on classified information.'

Capabilities: After translating secret military documents, forums and studying images of a network of tunnels in China, the students believe the country could have as many as 3,000 warheads

Might: China's military has been expanding rapidly and in 2009 it admitted the existence of 3,000 miles of underground tunnels

Critics have questioned the study's use of internet research, including a fictional TV show about Chinese artillery soldiers, and fuelled concerns it could encourage nations to maintain nuclear arsenals as a deterrent.

The students' professor, Phillip A. Karber, was a former Cold War strategist who reported directly to the secretary of defense.

Three years ago, he was working on a committee for the Defense Threat Reduction Agency aimed at countering weapons of mass destruction.

Then after the 2008 earthquake Mr Karber enlisted the first students from his class at Georgetown to start looking into China's nuclear weapons - using military journals, news reports, photos and forums.

Students spent hours scanning pages which could then be converted into Chinese characters and eventually translated.

Force: New recruits for the Chinese People's Liberation Army stand at attention in Suining city. The study also shows how the military transports missiles through its tunnel network

Concerns: A Chinese naval vessel fires missiles during an exercise off the Shangdong peninsula

After three years of graft, they had created a searchable database of more than 1.4million words on the secretive tunnel system.

Among their findings, they were able to establish a rough guide of the location of the tunnels and the types of missiles inside them.

They were also able to work out how the missiles were transported, including the possible existence of a 'missile train' and modified railway carriages used to carry the weapons.

Mr Karber told the newspaper: 'I don't have the slightest idea how many nuclear weapons China really has, but neither does anyone else in the arms-control community.

'That's the problem with China - no one really knows except them.'

Underground: The study began after the 2008 earthquake in Sichuan province revealed strangely collapsed hills that were believe to have tunnels underneath them

In 2009, the Chinese military admitted the existence of the network of tunnels - 3,000 miles of them - including underground bunkers capable of withstanding nuclear attacks.

For years, the Chinese government has insisted it maintains a small stockpile of warheads for 'minimum deterrence'.

But the group of students say that number could in fact be as many as 3,000.

Gregory Kulacki, a China nuclear analyst at the Union of Concerned Scientists, told the Washington Post: 'The fact that they're building tunnels could actually reinforce the exact opposite.

'With more tunnels and a better chance of survivability, they may think they don't need as many warheads to strike back.'