The blast was picked up by seismic stations all over the world, and it was big.
The Comprehensive Test Ban Treaty Organization, which monitors the globe for nuclear tests, said that its monitoring system had gone off-scale. The U.S. Geological Survey recorded a 6.3-magnitude earthquake, which was human-made. That's far larger than the seismic signature from the North's last test, conducted roughly a year ago. Here's what you need to know.
This was probably not an "ordinary" nuclear test
North Korea's previous nuclear tests have been in the tens of kilotons range. That corresponds roughly to a weapon the size of the ones used on Hiroshima and Nagasaki at the end of World War II. It's believed that the North's earlier tests were of nuclear weapons that use uranium or plutonium (or both) for their explosive yield.
This time, the North claims to have mastered a far more powerful hydrogen weapon. Some early estimates are putting this test in the hundreds of kiloton range.
"That's very roughly the equivalent 100,000 tons of TNT. Maybe its 20 percent smaller, maybe 30 percent larger," says James Acton, a physicist and co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace.
Much analysis remains to be done, he notes.
And if it's a hydrogen bomb, as the North claims, that's a big deal
"Normal" nuclear weapons depend on the splitting apart, or fission, of atoms for their explosive power. But for thermonuclear weapons, that's just the start.
A comparatively small fission bomb made of uranium or plutonium is used to trigger a far larger blast through nuclear fusion, the sticking together of light atoms. Those atoms are hydrogen isotopes, which is where the hydrogen bomb derives its name.
Modern nuclear weapons of the sort possessed by the U.S. and Russia are almost all thermonuclear in nature. It allows the weapons to pack a huge punch while fitting in a warhead small enough to be delivered by a missile. These weapons are also tens or hundreds of times more powerful than the ones used at the end of the Second World War.
For his part, Acton says he says no reason to doubt the North Korean claim. "I don't have any serious doubt in my mind that this is what the North Koreans say it is, and that's a thermonuclear weapon."
Kim's nuclear photo op spoke volumes
A day before the test, North Korea released photos of Kim Jong Un posing with a hydrogen bomb, or at least a model of a hydrogen bomb. The photo shows an oblong device that looks a little like a dumbbell. One of the spherical shapes depicted could be the fission "primary" of the weapon; the other the fusion "secondary".
Whether or not the device in the picture is real, the photos are meant to clearly show that North Korea understands the concepts behind modern thermonuclear weapons. A diagram behind Kim also shows how the weapon could fit into a missile warhead.
Earlier this year, North Korea conducted two tests of an Intercontinental Ballistic Missile.
Military options are probably not the greatest idea
In a series of tweets following the test, President Trump opined that the North Koreans "only understand one thing!"
However, many analysts believe that even a limited military strike on North Korea could be potentially catastrophic. "No military analyst believes that we would be able to destroy all of North Korea's nuclear weapons or capabilities," says Jon Wolfsthal, a former adviser to Obama's National Security Council who now works with GlobalZero, an arms control nonprofit. "If you're not assured you've got them all, you're triggering a nuclear response."
And the North has thousands of conventional artillery pieces within range of the South Korean capital Seoul. That would mean a devastating nonnuclear response would also be possible.
What happens now?
Nobody knows. Much will depend on how the U.S. and allies in the region respond to the test. Additionally, China may be more willing to apply sanctions and other pressure on the North Korean regime.
Perhaps the greatest unknown is what Kim Jong Un will do with his nuclear arsenal. Traditionally, nations have used these weapons as a way to deter attacks by others, and so far, the young leader's actions seem to suggest he wants to preserve his power, according to Wolfsthal.
But, he adds, "The problem with deterrence is that it works up until the point it doesn't."
LULU GARCIA-NAVARRO, HOST:
There's been a major escalation in the world's standoff with North Korea this morning. The regime claims to have detonated a hydrogen bomb - one that could be mounted on an intercontinental ballistic missile. It's still unclear whether or not Pyongyang statements are true. With us to assess the claims is NPR's science editor Geoff Brumfiel. Good morning, Geoff.
GEOFF BRUMFIEL, BYLINE: Good morning, Lulu.
GARCIA-NAVARRO: All right, this seems like a very big deal. What makes this nuclear test different than the others?
BRUMFIEL: Well this is the sixth nuclear test, and it's by far the largest. In fact, the Comprehensive Test Ban Treaty Organization, which monitors these tests, said it went off scale on their equipment. They're usually designed to monitor smaller tests than this. And that raises the very real fear that this could have been the test of a thermonuclear or hydrogen device as North Korea claims.
GARCIA-NAVARRO: Why does it matter that they've claimed to have tested a hydrogen bomb?
BRUMFIEL: Well, believe it or not, not all nuclear bombs are created equal. So up until now, we believe North Korea has been testing so-called fission bombs that use uranium and plutonium - those are similar to the ones we used at the end of World War II on the Japanese cities of Hiroshima and Nagasaki.
Since then, nuclear weapons have come a long way. And hydrogen bombs are actually far more powerful weapons. They're the sort that the U.S. and Russia possess. They are advanced weapons that can destroy whole cities.
GARCIA-NAVARRO: And I guess this links with the fact that the North has also been testing missiles. Where are they with that?
BRUMFIEL: So in July, the North conducted two tests of intercontinental ballistic missiles that are basically capable of reaching the continental United States. They have shown pictures now of this new weapon they claim to have tested today. And it looks like it's possible it could fit on top of one of those missiles. They sort of show a mockup of it that looks like that could work.
So if what they say is true - and there's some indication that it is - then they have a powerful weapon. They have a powerful missile. Put it all together. And they could potentially hold the U.S. at risk. Not all the technology has been proven. We don't know exactly where it is. But it looks pretty serious.
GARCIA-NAVARRO: Just briefly, the president sent out a series of tweets this morning condemning the test. He also said they only understand one thing, presumably speaking about North Korea. Are there any good military options?
BRUMFIEL: In a word, no. There are absolutely no good military options right now. I mean, for one thing, it's not clear even if you tried to take out all the North's nuclear weapons, that you could. But even in some magical world where you can find them all and destroy them all right away, they still have enough conventional power to do massive damage to our allies. So it really is a tight situation that the U.S. is in.
GARCIA-NAVARRO: All right, that's NPR's science editor Geoff Brumfiel. Thanks so much.
BRUMFIEL: Thank you. Transcript provided by NPR, Copyright NPR.