A tunnel collapse at North Korea’s Punggye-ri nuclear test site has killed as many as 200 workers, according to regional media reports.

Around 100 people were reportedly working inside the tunnel when it collapsed, and another 100 are believed to have perished attempting to rescue the others, as a second cave-in occurred shortly after the first, reports South Korea’s Yonhap News Agency, citing Japan’s TV Asahi. It is unclear when the incident happened, but it is likely linked to North Korea’s most recent nuclear test.

The North detonated a suspected staged thermonuclear bomb — a hydrogen bomb — early last month, producing an explosive yield unlike anything previously seen in past North Korean nuclear tests. The blast damaged part of the underground tunnel structure, caused landslides, and has triggered multiple earthquakes, leading observers to conclude that North Korea’s Punggye-ri nuclear test site may be tapped out. 

A prevailing theory among some experts is that North Korea’s nuclear test site may be on its last leg, as instability could expose the North and its neighbors to fallout and other contaminates were Pyongyang to push forward with another nuclear test. There are also concerns that another test could cause the nearby volcano to erupt violently. 

The Chief of South Korea’s Meteorological Administration warned Monday that another nuclear test could cause a major collapse, resulting in a leak of radioactive material. Chinese scientists reportedly issued a similar warning to their nuclear neighbors, urging them to conduct their nuclear tests elsewhere to avoid a catastrophic disaster.

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