What do we know about the mysterious Punggye-ri test site? Chuck Tinte has the details. 

Situated in mountainous terrain in the north-east of the country, the Punggye-ri test site is thought to be the DPRK's main nuclear facility and the only active nuclear testing site in the world. The country has conducted six nuclear tests since 2006 and used the Punggye-ri site every time.

In October 2006, Pyongyang announced it had carried out its first nuclear test. International observers estimated the blast had an explosive force of about a kiloton, less than a tenth of the size of the bomb dropped on Hiroshima in 1945. The second test in May 2009 was bigger, with an estimated yield of between two and eight kilotons. Three years later, the DPRK said it had tested "a miniaturised and lighter nuclear device with a greater explosive force than previously".

In January 2016, Pyongyang said it successfully conducted a hydrogen bomb test. H-bombs, also known as thermonuclear warheads, are massively more powerful than atomic bombs.

The DPRK announced its fifth nuclear test in September 2016. Estimates of the explosive yield were varied. South Korea's military said it was about 10 kilotons, but other experts said indications suggested 20 kilotons or more.

A year later, Pyongyang claimed to have successfully tested a miniaturised hydrogen bomb that could be loaded on to a long-range missile. The US Air Force Technical Applications Center said an early yield estimate ranged between 70 and 280 kilotons, potentially more than 10 times larger than the last time. The DPRK state media called it a "very meaningful step in completing the national nuclear weapons program".