Japan's TV Asahi reports that about 200 North Koreans have died in a tunnel collapse at a nuclear test site in Punggye-ri, in North Korea's northeast.
In early September, North Korea conducted its most powerful nuclear test there, detonating a nuclear device under a mountain. Experts have said it was a hydrogen bomb about 10 times as powerful as the first atomic bombs dropped on Japan at the close of World War II.
Satellite imagery has revealed that the mountain above the test site has since suffered a series of landslides and seismic aftershocks thought to have resulted from the blast.
North Korean sources told TV Asahi that a tunnel collapsed on 100 workers and that an additional 100 who went in to rescue them also died under the unstable mountain.
Using the slider below, you can see the affects of the test detonations, especially along the mountain ridges:
The tunnels in and out of the test site had been damaged, and the workers may have been clearing or repairing them to resume nuclear testing.
If the test site is compromised, hazardous radioactive material left over from the blast may seep out.
If that debris were to reach China, Beijing would see that as an attack on its country, Jenny Town, the assistant director of the US-Korea Institute and a managing editor at 38 North, previously told Business Insider.