Chinese geologists say North Korea's main nuclear testing site has collapsed under the stress of five successive bomb tests.
A mountain above the notorious facility has collapsed, a study found, rendering it unsafe for future testing and presenting a serious risk of radiation leaking.
The paper's authors, Tian Dongdong, Yao Jiawen and Wen Lianxing, urged that authorities "continue to monitor any leakage of radioactive materials" in view of the "research finding that the North Korea nuclear test site at Mantapsan has collapsed".
The findings follow an announcement by President Kim Jong Un that North Korea would cease activity on its weapons testing programme.
The commitment to close the facility, made on Saturday, was welcomed by Donald Trump as "big progress", but Pyongyang has not suggested it will give up its nuclear ambitions entirely.
It also comes days before a historic summit between North and South Korea, where they are set to discuss the nuclear ambitions of North Korea.
North Korea's Kim Jong Un and the South's President Moon Jae In will meet at the border between the two countries tomorrow.
The secretive state has taken what appear to be historic and unusual steps toward conciliation in recent weeks, telling the US directly that it is prepared to discuss denuclearisation.
Study
The peer-reviewed study carrying news of the collapse is by China's university of science and technology, and collected data after North Korea carried out its most powerful bomb test to date on 3 September.
Such attempts have long been believed to have put the site at risk of collapse.
Geologists believe the explosion, with a yield estimated at 100 kilotons of TNT, may have triggered an initial collapse toward the test centre followed by an "earthquake swarm" over several weeks.
North Korean nuclear tests have been a cause for seismic concern in China before. The test site is less than 100km from the Chinese border and past radiation fears have prompted schools and offices to be evacuated.
Kune Yull Suh, a nuclear engineering professor at Seoul National University, has warned further tests could threaten volcanic eruption at Mount Paektu, on the North Korea-China border.
