Found this on my ATT (junk) home page. Digging into China Kind of makes you wonder what we have been paying our own intelligence services for, doesn't it?
Working for the DOD there are "pay grades" that as you go down people know less and less. There are many secrets even the president doesn't know until he needs too. My guess is the US knows they just don't tell anyone they know.
Perhaps they new something about these tunnels, but wouldn't you think at some point a 'leak' would have happened. Couldn't have just dug those tunnels during the past year.