Japanese media are reporting that a new tunnel under construction at North Korea’s Punggye-ri test site collapsed this month, killing up to 200 laborers.

The UK Telegraph notes that outside observers do not know the exact date of the collapse because the North Korean regime is not eager to discuss the incident, but it reportedly occurred after North Korea’s most powerful nuclear test detonation to date on September 3.

According to the Japanese report, the tunnel collapsed and trapped or killed about one hundred laborers who were working on expanding it. Another hundred were killed in a second collapse while attempting to rescue the first group.

As the Telegraph notes, the powerful September 3 blast reportedly collapsed several underground structures near Punggye-ri, as well as causing landslides on the surface.

Japan’s Asahi TV sourced its report to a North Korean official, who said the collapse occurred sometime around October 10. South Korean officials indicated they were aware of the report but could not confirm it.

Newsweek quotes experts who believe further testing in the existing Punggye-ri tunnels would risk a catastrophic collapse, but the Kim regime is unwilling to abandon the site entirely, so it is attempting to dig new tunnels to the north under the Mantapsan mountain. South Korean scientists informed their legislature on Monday that further nuclear tests by North Korea could not only trigger underground collapses, but risk spreading radioactive material into South Korea or China.

Chinese geologists have reportedly warned North Korea that further nuclear tests at Punggye-ri could cause the facility to collapse, although the Chinese Foreign Ministry would not confirm that such warnings have been formally delivered to Pyongyang.

“China cannot sit and wait until the site implodes. Our instruments can detect nuclear fallout when it arrives, but it will be too late by then. There will be public panic and anger at the government for not taking action,” a researcher from Peking University explained. Another researcher expressed fears that fallout from tunnel collapses after another blast could “spread to an entire hemisphere.”