North Korea's Hwasong-15 ICBM
North Korea's Hwasong-15 ICBMReuters

Few Americans have ever been to North Korea. Even fewer have been inside one of the country’s nuclear weapons facilities.

Siegfried Hecker, a professor at Stanford University and a former head of the Los Alamos National Laboratory, made his first private visit in 2004 and went back again and again.

He can only speculate why they gave him access to the country’s most sensitive nuclear sites. “What they wanted to tell the Americans is, ‘Okay, so you know we have … uranium capabilities to the bomb. And by the way, you’ll never know how much we have and you’ll never know where it all is.’”