As many as 200 North Koreans are presumed dead after underground tunnels at a nuclear test site collapsed, according to Japanese news reports.

The collapse at the Punggye-ri test site on Oct. 10 occurred while people were doing construction on the underground tunnel, Japan’s Asahi TV reported, citing a source in North Korea.

The television station also said that the force of North Korea’s sixth nuclear test on Sep. 3 probably contributed to the already unstable tunnel’s collapse.

The initial cave-in crushed around 100 people, and 100 others were likely killed when more tunnels gave way on top of rescuers.

Experts have been warning for more than a month that the test site was on the verge of collapsing since the nuclear blast. North Korea said it had detonated a hydrogen bomb, calling it a “perfect success.” It was the country’s most powerful bomb tested to date, and the blast was reportedly 10 times more powerful than the nuclear bomb that was dropped on Hiroshima at the end of World War II.

It’s not clear when the collapse occurred, but on Oct. 17, the website 38 North speculated whether Mount Manhap, where the regime’s nuclear tests are conducted, may be suffering from “tired mountain syndrome,” due to repeated underground nuclear tests that trigger seismic events.

Three days later, the Washington Post reported that imagery from Airbus satellites showed sections of the mountain had physically shifted before and after the test.

“What we are seeing from North Korea looks like some kind of stress in the ground,” seismologist Paul Richards told the paper. “In that part of the world, there were stresses in the ground, but the explosions have shaken them up.”

Yonhap quoted the chief of South Korea’s weather agency as saying another underground nuclear test could lead to the collapse of the test site and the release of radioactive material.