In the early hours of Wednesday morning, international monitoring stations reported unusual seismic readings emanating from North Korea.
The US Geological Survey reported a 5.1 magnitude earthquake near Punggye-ri, a site where the secretive communist country has carried out previous nuclear tests.
About an hour later, North Korean state TV announced the North had successfully tested a miniaturised hydrogen bomb.
What is a hydrogen bomb?
Many of us think of “splitting the atom” or nuclear fission when we think about atomic bombs.
That’s how the bombs dropped on Hiroshima and Nagasaki in 1945 worked: by suddenly splitting the nucleus of an atom of uranium or plutonium, releasing enormous amounts of energy.
By the early 1950s physicists were using nuclear fusion instead – detonating a fission “trigger bomb” to smash atoms of hydrogen or other elements into one another at high speed, creating a new nucleus and a second explosion.
This releases even more energy. In 1961 Russia’s AN602 “Tsar bomb” created a blast yield equivalent to 50 megatons of TNT – about 3,000 to 4,000 times more powerful than the device dropped on Hiroshima.
An aerial view of the A-bomb attack on Hiroshima in 1945 (Getty)
The arsenals of the major nuclear powers – the US, Russia, Britain, France and China – are thought to consist entirely of thermonuclear weapons.
Does North Korea really have one?
We don’t know. Scientists will be studying seismic and other readings as well as any radioactive material that emerges from the blast, but we may never know.
For now, all we have is a claim from North Korea, which has been known to exaggerate its technological achievements in the past.
Experts are mostly sceptical that the bomb detonated this morning was a traditional two-stage thermonuclear device, because the yield wasn’t big enough.
James Acton from the Carnegie Endowment for International Peace said an estimate of 10 kilotons was “a reasonable order-of-magnitude yield”.
South Korea’s spy agency said the blast was probably even weaker – around 6 kilotons, less than what you would expect a failed thermonuclear bomb to produce.
The estimates of yield and seismic activity are very similar to the last nuclear bomb test North Korea carried out in 2013, when they made no claims of the device being an H-bomb.
Several atomic weapons experts have said the bomb could have been a “boosted fission” weapon, which uses a small amount of fusion fuel like hydrogen to increase the yield of a traditional Hiroshima-style device.
Who can North Korea hit with nukes?
Even if Pyongyang has developed a working thermonuclear bomb, there are still big questions about whether it has the technology to use it as an effective weapon.
To pose a real threat, you need to make the bomb small enough to fit in a missile, and the missile has to be capable of hitting long-range targets.
There’s a broad spectrum of opinion on whether North Korea can do either of these things.
In 2012 the country’s military showed off six new KN08 long-range intercontinental ballistic missiles mounted on massive carriers at a military parade.
Experts soon dismissed them as mock-ups rather than working missiles.
But recent analysis suggests the North may have developed a simpler, more reliable version with a range of roughly 5,500 miles, enough to hit the west coast of the United States.
Various arms of the US defence and security community seem to have different opinions on whether the KN08 could deliver a nuclear warhead to the continental United States.
A spokesman for the US National Security Council was quoted last year as saying: “We do not think that they have that capacity.
“However, they are working on developing a number of long-range missiles, including intercontinental ballistic missiles, that could eventually threaten our allies and the homeland.”
A declassified report from the Defence Intelligence Agency said it “assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles”. It added: “However, the reliability will be low.”
But Admiral Bill Gortney from the Department of Defense said last year: “Our assessment is that they have the ability to put it on – a nuclear weapon on a KN08 – and shoot it at the homeland.”
He also said he was certain the US could shoot down any missile fired by Pyongyang, saying: “I own the trigger on this, and I have high confidence that it will work against North Korea.”
What about conventional weapons?
Few North Korea-watchers think its leaders would really risk open war with the old enemy South Korea or her allies, nuclear or otherwise.
But analysts keep a close eye on the country’s military and its arsenal of conventional weapons.
The latest assessment by the International Institute for Strategic Studies (IISS) suggests that the North could mobilise around 1 million soldiers, making it one of the largest armies in the world, supported by 3,500 tanks and more than 500 combat aircraft.
North Korea also has a formidable artillery presence, much of it massed menacingly close to the border with the South.
But there are doubts about the quality of the equipment Pyonyang could put in the field in the event of a conventional war.
On paper, its fleet of 70 submarines is the second-largest in the world, but detailed analysis from IISS suggests most of the subs are either obsolete Russian machines or North Korean-made vessels of questionable quality.
Researchers at BBC Monitoring think the most modern fighter planes flown by North Korea’s air force are Soviet-era Mig-29s dating back to the late 1980s, with perhaps only a handful of these still fit to fly.
The US Department of Defense says the North “fields primarily legacy equipment, either produced in, or based on designs of, the Soviet Union and China, dating back to the 1950s, 60s and 70s, though a few systems are based on more modern technology”.
But even old weapons can still be deadly, and North Korea has sunk South Korean ships and shelled its neighbour’s territory in recent years.
The American assessment concludes that “the North Korean military poses a serious threat to the Republic of Korea, its other neighbors, and US forces in the region”.
Should we panic?
No, although experts are concerned at the latest bomb test, and what it suggests about international efforts to restrain North Korea’s nuclear ambitions.
The UN and individual countries have hit North Korea with sanctions in the past for its nuclear tests. In 2012 the US cancelled a food aid programme after Pyonyang launched a satellite into orbit.
Even the country’s closest ally, China, has expressed its displeasure over bomb testing.
Timothy Stafford from the Royal United Services Institute think-tank told us: “The big takeaway from today is that western policy has failed.
“Clearly, the North Koreans are not interested in nuclear non-proliferation. They are prepared to run the risk of sanctions. They are pursuing it to the extent that even when the Chinese want them to stop, they don’t.
“This is another quite significant step on the road that they were on already. That particular path does end up in quite a problematic situation. All things remaining equal, they will develop a working nuclear weapons scope and the ability to deliver it far and wide.”
Dr Heather Williams from King’s College London’s Centre for Science and Security Studies said: “With regard to North Korea, there really are no easy options.
“If there are more sanctions, the people who are going to feel the brunt of that are not the Kim regime, but the North Korean people.
“The best course of action is to continue to stigmatise North Korea and make them a pariah of the international community.”