(BEIJING) — A study by Chinese geologists shows the mountain above North Korea’s main nuclear test site has collapsed under the stress of the explosions, rendering it unsafe for further testing and necessitating monitoring for any leaking radiation.

The findings by the scientists at the University of Science and Technology of China may shed new light on North Korean President Kim Jong Un’s announcement that his country was ceasing its testing program.

Nuclear explosions release enormous amounts of heat and energy, and the North’s largest test in September was believed early on to have rendered the site in northeastern North Korea unstable.

The data in the latest Chinese study was collected following the most powerful of the North’s six nuclear device tests on Sept. 3 that is believed to have triggered four earthquakes over the following weeks. The yield of the bomb was estimated at more than 100 kilotons of TNT, at least 10 times stronger than anything the North had tested previously. (The bomb the United States dropped on Hiroshima in 1945 had a yield of about 15 kilotons.)

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