chris lawrence has a closer look at what is being called time the's underground great wall -- undergrounda's great wall. >> while china was digging these tunnels, this professor was digging into china. they have produced the world's largest report on china's tunnels. china admits there were dug by a secretive branch of the military, responsible -- they were dug by a secret a branch of the military, responsible for deploying warheads. >> 3,000 miles of tunnels. >> can you put that in perspective? >> imagine a tunnel 30 feet by 20 feet high, running from nova scotia to tijuana. >> he is a former pentagon strategist who used to look for weaknesses in the old soviet union. he says china could have as many as 3000 nuclear warheads. >> the problem with the steady and the way that it comes to this estimate is that the students and their professor make the assumption that, because china is working on this system of underground tunnels, this must automatically mean that they have -- they are working on new nuclear weapons. >> this policy analyst and others working on arms control questioned the georg