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:

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:

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:

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

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.