Like trampling on a crime scene

Bruce Bechtol says evidence that might have been collected will now be lost to the world.

"It's kind of like a murder scene where they let people like you and me trample around in it, it's the same concept," said Bechtol, a professor of political science at Angelo State University who has authored several books on North Korea.

"The North Koreans have conducted all these tests here, every single weapons test, so if they let experts in to look at these tunnels before they let anyone else in that would be potentially for us an intelligence boon," he said.

Some observers have said that the site had become partially unusable anyway due to the damage incurred after six nuclear tests since 2006, while others say the site was still in operation only months ago.