North Korea learned how to test nukes from the U.S.

North Korea tests its nuclear bombs in a valley beneath a cluster of mountain peaks about 370 miles northeast of Pyongyang. And at least three times over the past month, teams of soldiers at the heavily secured compound have been spotted playing volleyball.

The games, captured by commercial satellites passing high above, have generated headlines around the world — and more than a few “Top Gun” jokes. But the workers at the remote testing site have been busy with far more than just leisure activities.

Satellite imagery of the site, which is named after the nearby village of Punggye-ri, has revealed signs of fresh tunneling, new equipment, and other activities known to have preceded North Korea’s five former nuclear tests, two of which occurred last year. President Donald Trump says he wants the tests stopped, but the Kim Jong Un regime appears to have no intention of letting up. And Jeffrey Lewis, who leads East Asia research at the James Martin Center for Nonproliferation Studies, says there’s evidence that North Korea “will probably conduct one or two nuclear tests every year for the foreseeable future.”

“They’ve started economizing and building basically underground test centers where they can do many, many tests,” Lewis said. “And that suggests something distressing about their aspirations. They’re in this for the long haul.”

That forecast is based in part on the discovery that North Korea likely copied the design of its nuclear testing site from the United States. Lewis is one of a handful of open-source sleuths who use satellite imagery, seismological data, propaganda photos, and other publicly available information to study North Korea’s development of weapons of mass destruction. Last year, when mapping the precise locations of North Korea’s previous underground nuclear tests, he and his colleagues noticed something peculiar: The layout of the tunnels almost perfectly matches the configuration at the Nevada National Security Site, where the U.S. conducted many of its own nuclear tests.

“I would be shocked if this was a coincidence,” Lewis said. “The U.S. has done more than a thousand nuclear tests, and very many of those were underground. That data is freely available. And my suspicion is the North Koreans make good use of that data.”

The government has indeed declassified info about America’s old nuke tests; there are even books on Amazon that describe how the underground tunnels were designed and spaced to keep nuclear blasts safely contained. In North Korea, each test creates a small earthquake, and there are standard formulas to calculate the distance between underground explosions using seismic readings. Lewis and his team created interactive 3D maps of Punggye-ri’s tunnels [see below] and overlayed them with the U.S. setup in Nevada. “It turns out you could almost perfectly just drop it right on top,” he said.

Kim Jong Un’s father began building the Punggye-ri site in the early 2000s, razing several small villages in the process. The site originally had just one tunnel, but now, Lewis says, it’s “basically a base camp surrounded by big mountains with three tunnels going into different mountains.”

The test site is one of the most closely watched pieces of real estate in North Korea, with intelligence agencies and open-source researchers scrutinizing satellite photography on a near-daily basis for clues about the likelihood of another test. Some of the most influential observations come from 38 North, a website affiliated with Johns Hopkins University. When experts Joe Bermudez and Jack Liu wrote in April that the site appeared to be “primed and ready” for another test ahead of a major North Korean holiday, news outlets around the world, including VICE News, picked up the story, which turned out to be a false alarm.

Bermudez, who is based in the Denver area, says crying wolf is “one of our biggest concerns.” He maintains that it appears “everything has been done should they want to conduct another test,” but it’s basically impossible to predict when that will happen.

“That decision is in one person’s hands and one person’s alone: Kim Jong Un,” Bermudez said. “He has to sign a special order for the test to be conducted. He could wake up this morning and say ‘Go.’ He could wake up this morning and say ‘No, let’s wait until the U.S. tones down its rhetoric and the U.S. aircraft carrier leaves the area, and once they leave, we’ll pop the nuke.’”

Bermudez used to work for the government in some capacity — he’s cagey about his exact role — and has studied North Korean defense and intelligence affairs for more than three decades. He was the one who noticed the recent volleyball matches. The sport, he pointed out, is popular across North Korea and not unprecedented at the testing site, with games observed there as far back as 2006, shortly after the first underground blast.

In a Skype chat, Bermudez shared his computer screen with VICE News and spent 45 minutes using two high-resolution satellite photos to explain the layout of the testing site and point out some oddities that can be observed from space. For instance, there are several greenhouses inside the compound because workers are expected to grow some of their own food.

“They’ve actually started planting for spring,” Bermudez said. He can’t see into the greenhouses, but he uses software that manipulates the image to highlight vegetation. After a few clicks, the opaque image of the greenhouse is full of little pink spots. “What we’re probably seeing in this pinkish shade is the beginning of seedlings that are being grown under the plastic,” he explained.

Bermudez also noted a row of bright white dots near the volleyball courts he suspects are propaganda placards. He says there appears to be a fresh coat of paint on them, a sign that a test could come soon. “Before a major event or a visit by a senior official, they’ll have what we call a beautification program, and this is part of it,” he said.

Other indications include the presence of machines that pump water out of the testing tunnels, and the accumulation of equipment piled up under tarps and canopies. The North Koreans know they’re being watched from space and frequently try to hide their work and confuse outside observers.

Bermudez wrote that a recent day of bustling activity at the site — including plenty of volleyball — was likely a “deception and propaganda effort” to fuel curiosity and concern about the likelihood of another nuke test. If that was the true intent, it worked like a charm. After the frenzy of media coverage generated by Bermudez’s 38 North posts in April, Trump weighed in, saying he would “not be happy” if Kim orders another nuke test and wouldn’t rule out taking military action in response.

Lewis doesn’t like predicting when the next test will occur. But based on his research, it appears North Korea has enough space underneath the mountains to dig more tunnels and conduct many more nuclear tests. So far, the explosions have been relatively small; the largest was estimated to be roughly the same size as the one generated by the bomb the U.S. dropped on Hiroshima — though Lewis suspects the depth of the blast may have concealed its true size. Based on the designs North Korea borrowed from the U.S., Lewis says it appears the regime has the capacity to detonate a weapon about 10 or 20 times more powerful than what they’ve done thus far.

“It’s pretty clear,” Lewis said, “that they plan to do a lot of nuclear tests.”

Ravi Somaiya and Amel Guettatfi contributed reporting.