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I sort of figured something like this must be going on:

A large part of North Korea’s underground nuclear test facility is unusable due to the collapse of a cavity inside the mountain after the latest test-detonation occurred, according to Chinese seismologists involved in a soon-to-be-published study. ….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….The researchers warned that a nuclear test of similar yield to September’s “would produce collapses in an even larger scale creating an environmental catastrophe,” according to the abstract.

In other words, Kim Jong Un’s big announcement that North Korea was suspending nuclear tests and closing its test site were pretty much meaningless. It was something he had to do regardless, and it will probably take another year or two to build a new one.

UPDATE: Jeffrey Lewis thinks this whole thing is overblown:

No, North Korea’s nuclear test mountain did not collapse. (A note in place of the short thread I will write after I drop my children at school.) — Jeffrey Lewis (@ArmsControlWonk) April 25, 2018

Click the link to read the entire thread. Nickel summary: one cavern collapsed, but there are others.