North Korean leader Kim Jong Un's promise to dismantle its only known nuclear test site next month has shone the spotlight on the secretive facility near the Chinese border.
The Punggye-ri test site, located beneath a mountain in the country's northeast, has hosted all six nuclear tests Pyongyang has conducted -- most recently last September.
Attention is now focused on whether that will prove to be the final test, as the diplomatic push for Pyongyang to give up its nuclear weapons gathers pace ahead of Kim's eagerly-awaited summit with US President Donald Trump.
The site is located deep inside mountains in the northeastern province of North Hamgyong, which borders China.
Surrounded by high, craggy peaks and carved deep into a granite mountain more than 2,000 metres high, the test site is said to be an ideal venue to withstand the huge forces unleashed by nuclear blasts.
The presence of the site became known in 2006 when the North conducted its first nuclear test under Kim's late father, longtime ruler Kim Jong Il, with activities closely watched through satellite imagery since then.
Tunnels can be seen entering the site from different directions. The first test was staged in the eastern tunnel, the second and third in the western tunnel and the remainder in the northern tunnel, according to intelligence authorities.
Tests staged at the site have demonstrated the country's rapid progress in its nuclear programme -- especially since Kim took power in 2011 and oversaw four atomic tests in only six years.
The country's first test was largely seen as a failure and produced an estimated yield of only about a kiloton, compared to as much as 250 kilotons in the sixth -- an explosion more than 16 times powerful than the US atomic bomb dropped on Hiroshima in 1945.
But Punggye-ri's proximity to China has become a source of concern for Beijing, as the tremor from the sixth test was felt across the border and prompted many residents to flee their homes in panic.
The growing impact of the blasts raised safety concern at the Punggye-ri site, with some Chinese scientists warning that it could pose a major radioactive threat to the wider region.
Potential damage to the site became a topic for debate again this month after Kim declared he would shut down the facility -- followed by Sunday's revelation that he would invite foreign observers to verify it in May.
Sceptics have said it was an empty concession by Kim as the site is already suffering from "tired mountain syndrome" and may be obsolete.
A recent study by seismologists at the University of Science and Technology of China also suggested rock had collapsed under the Mantap peak, making it unusable.
But Jeffrey Lewis of the Middlebury Institute of Strategic Studies dismissed the claims, saying there is "no basis" to conclude it is no longer usable and the promised closure is "not a case of passing off damaged goods".
The North has long claimed that its nuclear tests posed no environmental threats, saying there was "no radioactive leak" after conducting tests.
But some South Korean and Japanese media reported that workers at the site or area residents suffered from radioactive exposure and symptoms including cancer and the births of deformed babies, citing the North's defectors and researchers.
Such concerns prompted Seoul's unification ministry to run medical checkups on 30 defectors who hailed from the region for potential radioactive exposure last year.
Four of them -- from the county of Kilju that includes Punggye-ri -- showed symptoms that could be attributed to radiation exposure, but researchers involved in the study said they could not conclude that the health problems had been caused by a nuclear test.
