The underground nuclear explosion in North Korea on September 3rd, 2017 was so powerful that it moved a mountain, according to new research.

The blast was North Korea’s sixth and largest nuclear test, conducted in an underground tunnel beneath Mount Mantap about 400 miles northeast of Pyongyang, North Korea’s capital. North Korea’s leader Kim Jong-un claimed the explosion was the successful test of a hydrogen bomb. But it’s hard to get accurate information in or out of North Korea, so the world has been trying to reconstruct what exactly blew up and where from the vibrations that radiated from the test site.

The massive blast made the mountain bulge sideways

Now, a team of scientists has shown off a new way to investigate North Korea’s claims: by analyzing high-resolution satellite images that use radar to reconstruct the surface of the Earth. The images helped researchers pinpoint where exactly the blast occurred beneath Mount Mantap, which helped them narrow down the size of the explosion: roughly 13 to 16 times the size of the bomb dropped on Hiroshima in 1945, according to the study published today in the journal Science. The massive blast made the mountain bulge sideways by about 12 feet and collapse vertically by about a foot and a half, the study reports. “It’s almost pancaking,” says study author Roland Burgmann, a seismologist at the University of California, Berkeley.

These details help fill in gaps about a nuclear program we know little about. As President Donald Trump heads into negotiations with Kim Jong-un next month, it’s important to know how powerful and dangerous North Korea’s nuclear weapons are. “The radar information really seems to help create a more accurate picture of what happened at the test site,” says David Albright, president of the non-governmental Institute for Science and International Security, who was not involved in the study. Seismologist Steven Gibbons at the Norwegian Seismic Array, or NORSAR, agrees. “It’s important to have all the pieces we can get to put this jigsaw together,” says Gibbons, who also did not participate in the research.

We already had one piece of the jigsaw. Nuclear blasts can be picked up by the same sensors that measure earthquakes, and seismologists could tell immediately that whatever created the tremor on September 3rd was big. It shook the Earth with the magnitude of a 6.1 quake, according to the international watchdog that monitors nuclear testing around the world, called the Comprehensive Nuclear-Test-Ban Treaty Organization. Eight and half minutes later, there was another smaller tremor at the test site.

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By analyzing the types of seismic waves and the directions they traveled, seismologists were able to tell that the September 3rd tremor was caused by a nuclear blast. Researchers then estimated that the explosion’s size was equivalent to anywhere between 120,000 and 300,000 tons of TNT, or between nine and 23 times the size of the blast at Hiroshima. But that was a rough estimate, and there were still a lot of unknowns, such as where exactly the explosion started, how the blast changed the landscape around it, and what caused that second, smaller tremor.

Burgmann and his colleagues at UC Berkeley, Nanyang Technological University in Singapore, and other institutions turned to satellites that captured images of the test site from space in the days before and after the nuclear test. These pictures, taken by the German TerraSAR-X satellite, are a kind of high-resolution imagery that uses radar data to create detailed maps of the Earth’s surface. By analyzing the location of the pixels, the study authors pinpointed where the explosion started because all the pixels, and terrain they represent, moved away from their original spot. “It’s a really powerful tool,” Burgmann says. They also figured out that the mountain bulged sideways by about 12 feet, and shrank vertically by roughly a foot and a half.

“We were all really surprised.”

“We were all really surprised by how big the horizontal motions were and how little the vertical motions were,” says study author Doug Dreger, a seismologist at UC Berkeley. So what could be causing these changes? The team fed data collected by earthquake sensors into their simulation to figure out how much energy the blast released. They also guessed that the test site was made out of a type of granite-like rock. (That makes a difference in the way that vibrations from the blast travel through the Earth.)

Their results show that an explosion equivalent to between 171,000 and 209,000 tons of TNT went off in a tunnel about 1,500 feet beneath the summit of Mount Mantap. First, the explosion blew the mountain outward — melting and vaporizing the rock around the tunnel. Then, the mountain collapsed into the cavity left by the blast. Eight and a half minutes later, there was another tremor, likely caused by the implosion of the tunnel or cavity. But that wasn’t all: the mountain continued to slowly shrink as it settled into the cracks and crevices of the rocks crushed by the explosion.

Adding the radar data helps reduce some of the uncertainty that comes with reconstructing a nuclear test from afar, says Catherine Dill, a researcher at the Middlebury Institute of International Studies. “You’re always going to have some uncertainty, but they’ve really been able to narrow down a fair amount of that,” she says. Still, this is a model, so the team had to make assumptions about what the mountain was made out of, for example.

“You can only do so much remotely.”

That’s why Dreger cautions against jumping to conclusions about North Korea’s nuclear testing. Following the September 3rd test, news outlets had speculated, based on less-detailed satellite images, that the test site had collapsed. That could explain why Kim Jong-un recently promised to shutter it: if the test site is now unusable, that would mean he wasn’t actually giving up anything of value. But Dreger doesn’t think that his team’s results necessarily show that the test site was too destroyed to test there again. “The only people who really know how significantly the tunnel network may have been damaged are the scientists and technicians on site, people who have gone underground,” he says. “You can only do so much remotely.”