It was reported earlier this year that the mountain under which the base is believed to be hidden was at risk of collapsing and leaking radiation into the region.
Experts said if the peak crumbles, clouds of radioactive dust and gas would blanket the region, the South China Morning Post reported.
The Punggye-ri test site is carved deep into the side of Mount Mantap.
Geophysicist Wen Lianxing and his team at the University of Science and Technology of China in Hefei, Anhui province, said they were “confident” underground detonations were occurring underneath the mountain.
They posted an analysis of data collected from more than 100 seismic monitoring sites across China.
This has narrowed down the location of Pyongyang’s nuclear tests with a margin of error of just 100m. They’ve all been under the same mountain.
Seismic data showed the underground test triggered an earthquake of magnitude 6.3, around ten times more powerful than the fifth test a year ago.
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Satellite images showed the blast caused numerous landslides around the Punggye-ri test site, according to the Washington-based 38 North monitoring project.
But Chinese nuclear weapons researcher and chair of the China Nuclear Society Wang Naiyan told the Morning Post a collapse could spark a major environmental disaster.
He said: “We call it ‘taking the roof off’. If the mountain collapses and the hole is exposed, it will let out many bad things.
“A 100 kiloton bomb is a relatively large bomb. The North Korean government should stop the tests as they pose a huge threat not only to North Korea but to other countries, especially China.”
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Satellite photos taken just a day after the blast reveal new gravel and scree fields shaken loose by the blasts at an elevation of about 2205m.
Analysts said these appeared more numerous and widespread than those caused by previous detonations — which would be in keeping with the increased size of the bomb.
Wang said there are limited mountains in North Korea that are “suitable” to conduct a nuclear test.
He said if the North had simply drilled into the side of the mountain, this increased the risk of “blowing the top off”.
News of the tunnel collapse comes after it emerged Russia and the US have both flown nuclear bombers near the country as tensions grow over Kim’s nuke threats.
Why did North Korea test its H-bomb underground?
Nuclear devices are often tested underground to prevent radioactive material released in the explosion reaching the surface and contaminating the environment.
This method also ensures a degree of secrecy.
The release of radiation from an underground nuclear explosion – an effect known as “venting” – would give away clues to the technical composition and size of a country’s device.
How exactly does the underground test work?
A test site is carefully geologically surveyed to ensure suitability – usually in a place well away from population centres.
The nuclear device is placed into a drilled hole or tunnel usually between 200-800m (650-2,600ft) below the surface, and several metres wide.
A lead-lined canister containing monitoring equipment is lowered into the shaft above the chamber.
The hole is then plugged with gravel, sand, gypsum and other fine materials to contain the explosion and fallout underground.