North Korea set off a series of massive explosions at the Punggye-ri nuclear testing site in the northeast of the country, claiming it was destroyed. CBS News was the only U.S. broadcast network to witness it.

At the entrance to tunnel No. 2 at North Korea's nuclear test site, where they have conducted five of their six nuclear tests over the last couple of years, you could see they now had explosives strung up in there. They plan to blow this up so they can no longer use it.

With several large explosions, North Korea put on quite a show. It claims it has now destroyed all of the test tunnels at its infamous Punggye-ri nuclear testing site. That's where the regime conducted all six of its nuclear explosions beginning in 2006. Some outside scientists say the site was already so heavily damaged by previous tests that it was unusable. North Korea says that's not true.

CBS News witnessed a series of massive explosions at the Punggye-ri nuclear testing site in North Korea. CBS News

They say tunnel No. 3 is relatively new and that it's still usable, so rigging it with explosives and blowing it up is to them a real concession.

Until now, satellite photos are the only glimpses we have had of this highly secretive and heavily guarded site.

North Korea allowed us to walk right up to the nuclear test tunnels and said there was no concern about radiation. They say they have never detected any here at the site. But the one thing they did confiscate from our luggage was equipment to detect radiation.

Foreign journalists were told to keep the shades drawn on the train ride to North Korea's nuclear testing facility. CBS News

The journey to the remote mountainous site took nearly 15 hours, most of which we spent on this specially outfitted North Korean train. In the dining car, waiters wore white jackets and served an elaborate 10-course meal. But in the sleeping quarters, there were strict rules.

Government minders came by and said a shade had to be down the entire time. Apparently they didn't want us to see out the windows, to see perhaps how people are living in North Korea.

They did want us to see the explosions, but we can only confirm they blew up the entrances to the tunnels. If the tunnels inside the mountain still exist, this site could potentially still be used for future nuclear tests.