The reasons are ostensibly diplomatic, pointing to a thaw in relations between Kim Jong-un’s regime and South Korea and the West, but some noted that Pyongyang might have also been worried that the mountain was at risk of collapsing, as it visibly shifted during the last nuclear test. However, two separate groups of Chinese scientists now say Mount Mantap did in fact collapse after that detonation.

That means there’s a risk of radioactive contamination spreading not only within North Korea, but to other countries in the region. The site is not far from North Korea’s borders with China and Russia.

According to geologists from the University of Science and Technology of China, the collapse took place minutes after Kim Jong-un’s regime conducted its last nuclear test in September of last year.

The test of the 100-kiloton bomb, which led Chinese seismologists to register a 6.3 magnitude earthquake, apparently opened up a hole of up to 656 feet in diameter. Part of the mountain then fell into the hole.

The findings of the team, led by renowned seismologist Wen Lianxing, are set to be published next month in the journal Geophysical Research Letters.

According to the Hong Kong-based South China Morning Post, another team from the China Earthquake Administration reckons the collapse created a “chimney” that could allow the escape of fallout. The publication quoted researcher Zhao Lianfeng from the Chinese Academy of Sciences as saying the site was “wrecked” beyond repair.

So Pyongyang’s renouncement of land-based nuclear tests, for now, appears to be motivated by more than mere diplomatic concerns.