It was 10:36 a.m. on October 9, 2006, when South Korea's sensors first picked up the abnormal seismic waves.
Around half an hour later, experts started narrowing down the location of the epicenter. Eventually, they concluded it was Gilju County, an area in North Korea's northernmost province.
What Seoul's equipment was detecting was North Korea's first-ever nuclear test, carried out at its only known test site in the North Hamgyong region.
It would end up being the first of six, spanning over a decade, that have punctuated Pyongyang's journey to weaponizing nuclear material and insisting it will be recognized as a nuclear power on the world stage, despite nearly universal condemnation.
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Twenty years on from North Korea's first nuclear test, its military arsenal is ballooning and South Korea's worries about its secretive but bellicose northern neighbor are anything but soothed.
South Korean analysts quickly said their data showed the 2006 explosion was likely 0.8 kilotons, which is a small yield compared with the nuclear tests carried out by the U.S. or the Soviet Union. Russia, meanwhile, put the yield of the 2006 North Korean test explosion several times higher than Seoul's estimate, although most experts say it didn't exceed one kiloton.
The infamous Tsar Bomba test at Moscow's remote Novaya Zemlya site in 1961 had an estimated yield of 50,000 kilotons—more than 62,000 times the believed yield of Pyongyang's initial test.
Two weeks after the North Korean test, a monitoring station in northern Canada recorded higher levels of Xenon 133, the radioactive noble gas produced during nuclear fission.
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2006 was a turning point because it was the first time in eight years the world was contending with a new nuclear test, said William Alberque, a former head of NATO's nuclear non-proliferation center currently with the Pacific Forum nonprofit.
"It was a failure," he told Newsweek, "but they learned a lot."
Pyongyang had warned in the days leading up to the first test that it was planning to carry out a nuclear explosion, and state media hailed it as a "great leap forward" and a "historic event." International experts, meanwhile, said the test likely hadn't gone as planned, but also acknowledged this may have been a lower-yield trial than initially thought.
North Korea was then slapped with United Nations sanctions that have largely failed to halt Pyongyang's nuclear testing, nor cut off its ballistic missile development.
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Pyongyang has carried out five more nuclear tests in the past 20 years, including two within roughly nine months of one another in 2016.
The last test registered the largest magnitude, reaching an estimated 6.1, according to nuclear experts. It was picked up by over 100 monitoring facilities and months later, international teams had identified more than 30 smaller aftershocks around the test site.
North Korea is the only country to have carried out nuclear tests in the 21st century, and no confirmed nuclear tests have taken place anywhere in the world since 2017, despite President Donald Trump suggesting last year the U.S. would resume nuclear testing "on an equal basis" with other nations. The U.S. Department of Energy then clarified it wasn't planning on carrying out nuclear explosions.
The 2006 test shattered the widely-observed but unofficial pause on nuclear testing that was ushered in by the Comprehensive Test Ban Treaty opening up for signatures in 1996. The agreement hasn't been officially ratified, despite most countries abiding by it.
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Most of the thousands of nuclear tests—including all of North Korea's—have taken place underground. A 1963 treaty largely stopped tests in the atmosphere and underwater after concerns bubbled around radioactive fallout and nuclear material spreading in water and steam.
Since 1945, nuclear test explosions have taken place in all environments, from dug-out tunnels to on top of towers or hundreds of miles into the atmosphere.
Experts say tests underground have nevertheless leaked radioactive material into the atmosphere and contaminated the soil around the test sites.
Several months after North Korea's most recent nuclear explosion, the country's Supreme Leader, Kim Jong Un, used his New Year's address for a pointed new message.
"The entire United States is within range of our nuclear weapons," he said. "A nuclear button is always on my desk."
North Korea's Growing Nuclear Capability
Independent estimates put North Korea's stockpile at around 60 warheads, but the country could have enough nuclear material to make up to 90 weapons.
But different numbers have been floated by lawmakers. In August, South Korea said its neighbor could have up to 120 nuclear warheads—a count far above an assessment Trump made a day earlier, claiming Pyongyang had 57 nuclear weapons. In comments this month, Trump then appeared to say North Korea had 112 nuclear weapons.
As of 2026, North Korea has a host of missiles that can reach different targets, including a raft of intercontinental ballistic missiles (ICBMs) capable of reaching the U.S. mainland, although there are questions about whether Pyongyang would be able to reliably fire a nuclear warhead on these weapons.
"North Korea’s ICBMs can already reach U.S. soil," Tulsi Gabbard, the U.S.' then-director of national intelligence, said in March. "It is committed to expanding its nuclear arsenal."
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North Korea's first ICBM test in mid-2017 was quickly followed up by a trial of another ICBM design just four months later. ICBMs travel upward of 5,500 kilometers.
The Hwasong-17 and Hwasong-18 ICBMs are already operational, analysts say, with more designs under development.
Although there have been no recorded North Korean nuclear explosions for nearly a decade, Pyongyang has plugged away with trials of missiles which could carry nuclear warheads.
On October 3, North Korea said it had test-launched an intermediate-range strategic missile able to travel more than 1,000 kilometers. Pyongyang has overseen nearly a dozen missile tests so far this year.
Over the past 20 years, North Korea has developed better and longer-range missiles in large numbers, said Alberque.
International observers and U.S. government officials say North Korea has benefited from Russian help, particularly since the two countries inked a defense pact in mid-2024.
Pyongyang has offered up thousands of its soldiers, large shipments of ammunition and deliveries of ballistic missiles for Russia's war effort against Ukraine, gaining valuable data on how its missiles perform in combat when Ukraine's air defenses attempt to shoot them down. NATO has said Moscow in return is supporting North Korea's missile and nuclear programs.
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An Attack on the US?
The U.S. has for years said its missile defenses would be able to intercept an ICBM attack from North Korea.
The U.S. currently has 44 Ground-Based Interceptors (GBIs) deployed around the country, most of which sit in silos in Alaska. Four are housed at the Vandenberg Space Force Base in southern California.
The U.S. plans to add new missiles, dubbed the Next Generation Interceptors, to the GBIs in the next few years. Anything the GBIs don't knock out would likely fall to the U.S. Navy's Aegis system, analysts say.
North Korea doesn't have enough missiles to overwhelm American defenses, Alberque said. "They may build enough one day, but right now, I have a fairly high confidence they could shoot them down."
Yet as it stands, the U.S. does not have a joined-up system for intercepting large-scale ICBM strikes launched from Russia or China, with their far bigger stockpiles.
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The Trump administration is developing its so-called Golden Dome missile defense shield, which the government says will defend North America from advanced threats, like fast-traveling hypersonic missiles.
It is essentially a web of trackers, sensors and interceptors to knock out drones and missiles heading for the U.S., even ones launched from space. Much of the final design remains a mystery.
Countries are now "increasingly relying on nuclear weapons as instruments of national power—reversing decades of efforts to reduce the numbers and role of nuclear weapons," researchers with the Stockholm International Peace Research Institute (SIPRI) said in June.
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Twenty years after its first nuclear test, North Korea has transformed itself from a fledgling nuclear aspirant into a state with a growing arsenal, increasingly sophisticated missile technology and ambitions that continue to challenge efforts to curb nuclear proliferation.
Contact Newsweek editors on this story: John Fitzpatrick and Tony Phillips.