North Korea's nuclear programme has been a source of great concern for the international community for more than 20 years.
North Korea has conducted four underground nuclear tests so far.
Nuclear ambitions
Each one has taken it closer to what decades of international talks have tried to prevent - a nuclear weapon in the hands of one of the world's most unpredictable states.
The state is now believed to have conducted four underground nuclear tests.
The four tests
The latest test, announced on 6 January 2016, was described by North Korean media as a miniaturised hydrogen bomb test. There has been no independent confirmation of this, but it is the first claim of a hydrogen bomb, known to be immensely powerful.
9 October 2006 - a weapon for 'peace'
The previous test was conducted in February 2013 and came with the state's first claim to have miniaturised a device, although an atomic device in that instance.
Years of posturing - and attempts at negotiation by foreign powers - culminated in October 2006 with an announcement by Pyongyang that it had carried out its first nuclear explosion.
What this means is that it claims to have made a nuclear device small enough to fit on a missile. At the time, experts said it was impossible to confirm the claim.
Like all tests that would follow, it took place underground, in tunnels dug into a remote mountainous site called Punggye-ri, in the north-east.
Nevertheless, its nuclear ambitions have stepped up in the past year. Earlier this year, North Korea said its plutonium and highly enriched uranium facilities at nuclear plant Yongbyon had restarted.
The device is assumed to have used plutonium, sourced from the North's nuclear facility at Yongbyon.
The test site
International observers estimated the blast had an energy discharge of about a kiloton, less than a tenth of the size of the bomb dropped on Hiroshima in 1945.
All of the tests appear to have originated at a test site called Punggye-ri, also known as P'unggye-yok, in a remote area in the east of the country, near the town of Kilju.
Many believed this indicated a nuclear "fizzle" rather than a fully effective blast.
Multiple rounds of international negotiations amid a strict sanctions regime - a process, which has been described as a game of cat and mouse - appear to have done little to curb North Korea's nuclear ambitions.
But the North said it had joined the nuclear club and that its bomb would contribute to peace and stability on the Korean peninsula.
North Korea's previous tests followed rocket launches. The North put its first satellite into space with a long-range rocket in late 2012, in what the UN said was a banned test of ballistic missile technology, imposing sanctions.
25 May 2009 - attempt to prove stability
The US is concerned that such long-range rockets could threaten its mainland with atomic bombs.
The second test was bigger, with an estimated yield of 2-8 kilotons.
The key nuclear tests and developments
North Korea said it had achieved a "higher level in terms of its explosive power and technology".
9 October 2006
While the international consensus was that a nuclear test had happened, no radiation was detected. The ability to contain a nuclear test would in itself be a big advance for the North.
Years of posturing and wrangles over the delivery of American aid culminated in October 2006 with an announcement by Pyongyang that it had carried out an underground nuclear explosion.
It also followed hard on the heels of a test of a missile with a long enough range to reach the US.
North Korea's first nuclear test exploded a device based on plutonium, rather than enriched uranium.
Both were seen as an attempt by ailing leader Kim Jong-il to prove the North's nuclear capacity before he died.
The test was conducted at P'unggye-ri, and US intelligence officials later announced that analysis of radioactive debris in air samples collected a few days later confirmed the blast had taken place.
12 February 2013 - uranium-enrichment?
But they estimated that it had not been a powerful one, measuring less than one kiloton, raising questions about the sophistication and effectiveness of the weapon.
In the early hours of 12 February 2013, unusual seismic activity was again detected around Punggye-ri.
This is less than a tenth of the size of the bomb dropped on Hiroshima in 1945.
The North said it had tested "a miniaturised and lighter nuclear device with greater explosive force than previously".
In response to the test the UN approved military and economic sanctions against North Korea.
The as-yet unproven reference to "miniaturised" stoked fears that Pyongyang was closer to producing a device small enough to fit on a long-range missile.
After years of on-off talks, in February 2007 Pyongyang agreed to shut down its main nuclear reactor at Yongbyon in return for aid and diplomatic concessions.
Speculation was rife that the test involved a uranium device, though this has never been confirmed beyond doubt.
But negotiations stalled as North Korea accused its negotiating partners - the US, South Korea, Japan, China and Russia - of failing to meet agreed obligations.
A successful uranium test would mark a significant leap forward in North Korea's nuclear programme. The North's plutonium stocks are finite, but if it could enrich uranium it could build up a nuclear stockpile.
25 May 2009
Plutonium enrichment also has to happen in large, easy-to-spot facilities, whereas uranium enrichment can more easily be carried out in secrecy.
Barely a month after North Korea walked out of international talks on its nuclear programme, it carried out its second underground nuclear test, which was said to be more powerful than the first.
6 January 2016 - thermonuclear claim
Russia's defence ministry estimated a blast of up to 20 kilotons, a similar size to the US bombs that completely destroyed Hiroshima and Nagasaki in 1945.
The first indication was again report of an "artificial quake" in North Korea, registering about magnitude 5.1, close to Punggye-ri.
The US Geological Survey said a 4.7-magnitude quake was detected, 10km (six miles) underground, an indication of a nuclear explosion.
North Korea later announced it had conducted its first successful test of a hydrogen bomb.
It followed hard on the heels of the test of a long-range missile that threatened Asia and could have reached the US.
H-bombs, also known as thermonuclear warheads, are massively more powerful than atomic bombs, using fusion - the merging of atoms - rather than fission to unleash enormous amounts of energy.
The UN issued a resolution condemning the nuclear test and tightened sanctions. After the US imposed tough sanctions in August 2009, Kim Jong-il said he was willing to resume nuclear talks.
Though again it has never been confirmed, that claim alarmed the international community.
North Korea's rocket launches
Observations from afar suggested the blast detected was not large enough to have been a full thermonuclear device, but may have involved some nuclear fusion.
North Korea's missile programme
Again, North Korea said the device had been miniaturised.
November 2010: Tour of Yongbyon
9 March 2016 - a deliverable device?
In November 2010 a US atomic scientist was given a tour of North Korea's uranium enrichment plant at Yongbyon about 100km (60 miles) north of the capital.
A few months later, Kim Jong-un announced that North Korea scientists had been able to do what had long been feared, and make a nuclear warhead small enough to fit on a warhead.
Siegfried Hecker was shown an experimental light-water nuclear reactor that was still under construction, and more significantly a new facility that contained "more than 1,000 centrifuges" that the North Koreans said was processing low-enriched uranium to fuel the new reactor.
It came after days of threats to carry out "indiscriminate" attacks on the US and South Korea. But analysts said it was impossible to confirm.
He said the facility seemed designed primarily for civilian nuclear power but could easily be converted to further process uranium to weapons grade.
He described the plant as modern and clean - unlike all the other Yongbyon facilities he had seen - and said he was "stunned" at its sophistication.
12 February 2013
In the early hours of 12 February 2013, unusual seismic activity was detected around the Punggye-ri underground nuclear test site.
"It was confirmed that the nuclear test that was carried out at a high level in a safe and perfect manner using a miniaturised and lighter nuclear device with greater explosive force than previously," KCNA said.
The reference to "miniaturised" stoked fears that Pyongyang was that much closer to producing a device small enough to fit on a long-range missile.
But many experts remain sceptical. Rather, speculation was rife that the test involved a uranium device which could have significant implications for North Korea's nuclear capabilities.
However, a well-contained underground test could provide little evidence. Little was detected shortly after the test, although the Comprehensive Nuclear Test-ban Treaty Organization, compiled a paper about radiation detected 55 days later.
6 January 2016
The first indication was a geological report of a quake registering about 5.1 reported in North Korea, close to the nuclear test site.
Then KCNA announced that a successful test of a hydrogen bomb had taken place. It comes within a month of a claim by Kim Jong-un that they had an H-bomb.
Little data has been collected so far from this test, but the experts are sceptical about this claim.
