The mystery of the abrupt about-face in Pyongyang may have its answer deep beneath Mount Mantap. After years of belligerent threats to conduct nuclear tests until North Korea had weapons that could reliably hit the entire United States, Kim Jong-un suddenly reversed himself last week and declared an end to nuclear testing. The regime called further tests “unnecessary,” and suddenly began pursuing engagement with South Korea and the US rather than trading threats.

What changed? According to two studies authored by Chinese scientists reported by the Wall Street Journal, the North Korean nuclear weapons may have been too effective. Seismic activity and a sudden height change for Mount Mantap indicate that Kim’s underground test facility at Punggye-ri has collapsed, and that another test might be catastrophic for both North Korea and China:

A large part of North Korea’s underground nuclear test facility, which leader Kim Jong Un pledged to close, is unusable anyway due to the collapse of a cavity inside the mountain after the last blast there, Chinese scientists say.

Seismologists involved in a soon-to-be-published study also warned that another blast in the same spot and with similar yield could cause “environmental catastrophe.” …

Soon after the sixth and largest blast last September, satellite images suggested that one part of the site, a 7,200 foot granite peak called Mount Mantap had diminished in height. Some U.S. and South Korean experts suggested that tunnels inside the mountain—where five of North Korea’s six nuclear tests took place—had collapsed, rendering much of the site useless.

Now, the two Chinese studies give credence to that theory. They both used data from seismic monitoring stations in China and abroad to analyze the initial 6.3-magnitude tremor caused by the blast and another smaller tremor 8½ minutes later.

Both studies concluded that the second tremor, of 4.1-magnitude, was caused by the collapse of damaged rock above the blast cavity inside the mountain, rather than another explosion or a shift in tectonic plates.

This has been the subject of intense speculation ever since the test itself took place. The secondary seismic activity got picked up immediately and was assumed to be a collapse, as the yield was far greater than expected.  The primary quake registered a full point above the previous nuclear test:

The blast in September was the last test conducted at Punggye-ri, although there seems to be some debate over whether it remains functional. The Washington Post notes that a well-respected analytical site can’t decide either, despite access to satellite imagery:

38 North, a website that focuses on North Korea and frequently publishes satellite imagery, wrote in an unsigned commentary on Monday that Punggye-ri was “still, as far as we can tell, fully operational.” 38 North noted that although one part of the testing site did appear to be inactive — the North Portal — other parts still appeared to be functional.

The following day, 38 North published an analysis by Frank V. Pabian, Joseph S. Bermudez Jr. and Jack Liu that pointed to satellite imagery from Friday, the day after Kim’s announcement, showing at least eight mining carts and a new structure at Punggye-ri’s West Portal, a site not directly under Mount Mantap and not previously associated with nuclear tests but one with new tunneling seen as recently as early April.

The authors said this could be the “first visible indication that North Korea intends to cease further tunneling” but added that this was speculation and would require more assessment. It is not clear whether the tunneling was designed to get the area ready for more testing or why it had stopped.

There may be another explanation between the two. It’s a little curious that these China-backed studies have been allowed to go public. It might be that Beijing has had enough of the testing in part because further testing presents a very clear danger to their own territory. The South China Morning Post makes the danger of further use of Punggye-ri very specific:

The mountain’s collapse, and the prospect of radioactive exposure in the aftermath, confirms a series of exclusive reports by the South China Morning Post on China’s fears that Pyongyang’s latest nuclear test had caused a fallout leak.

Radioactive dust could escape through holes or cracks in the damaged mountain, the scientists said.

The “rock collapse … was for the first time documented in North Korea’s test site,” Liu’s team wrote in a paper published last month in Geophysical Research Letters.

The breakdown not only took off part of the mountain’s summit but also created a “chimney” that could allow fallout to rise from the blast centre into the air, they said.

And if that’s not bad enough, there’s also an active volcano in the region that could make matters infinitely worse:

“The test was not only destabilising the site but increasing the risk of eruption of the Changbai Mountain,” a large, active volcano at China-Korean border, said Hu, who asked that his university affiliation not be disclosed for this article because of the topic’s sensitivity.

If that’s the case, then Beijing may have made it abundantly clear to Kim that his nuclear-test days were over. There are other sites where Kim could set up underground test facilities, but (a) it would cost billions in hard currency that North Korea no longer can easily acquire, and (b) China has no reason to believe that Pyongyang wouldn’t create the same problem at a new site. Kim might be able to afford alienating Malaysia, but he can’t afford to cut ties with Beijing, not if he wants to continue his habit of breathing for very long.

If all this is true — and much of it is speculation — then Kim had little choice but to cash in his leverage and agree to denuclearization, hoping to get the best terms possible. That would actually be good news, as it means we have a real opportunity to settle the Korean question and dial down tensions in the region, rather than just play our usual part in show talks that go nowhere.

Kim will meet with Moon Jae-in on Friday. Perhaps Kim will tip his hand a little more in that summit, which will be televised live, at least in part. We’re not likely to see how much he’s bluffing until we call him on it in the Trump-Kim summit later this spring, though.