Update 12:05 p.m.:  The United Nations Security Council has finished their meeting regarding North Korea's nuclear test Tuesday and hit the country with ... a stern press statement. The AP reports: 

A press statement approved by all 15 council members at an emergency meeting hours after the latest underground test called the atomic blast a "grave violation" of three U.N. resolutions banning the North from conducting nuclear or missile tests.

Well, to be fair, it was a strongly-worded press statement and a promise to do more later: 

"In line with this commitment and the gravity of this violation, the members of the Security Council will begin work immediately on appropriate measures in a Security Council resolution," the council said.

We're not sure if this slap on the wrist and promise of more discussions going to anything to deter North Korea, a country that's impossible to negotiate with, from conducting more tests. 

Update 6:59 a.m.:  We're slowly getting more information on North Korea's third nuclear test. Here's what we know so far:

• It was probably plutonium. North Korea's Korean Central News Agency reported that they tested a smaller bomb. "The test was conducted in a safe and perfect way on a high level with the use of a smaller and light A-bomb unlike the previous ones, yet with great explosive power," reads the KCNA report on the test.  Analysts were really worried that North Korea was using a uranium-fueled weapon, which spells different kinds of trouble, like uranium being harder to detect, and it could possibly mean the involvement of of Iran. (Iran's foreign ministry spokesman said on Tuesday, rather dubiously  that "All weapons of mass destruction and nuclear arms need to be destroyed.") The smaller, device possibly indicates "that it had again used plutonium which is more suitable for use as a missile warhead," reports Reuters.
• Tuesday's explosion was twice as big as the last test. South Korea's defense ministry says that the explosion was around 10 kilotons. That's much bigger than the 2006 test, which yielded less than 1 kiloton, and around double its 2009 test which was between 2-7 kilotons. To put that in context, the bomb dropped on Hiroshima was about 10 kilotons. 
• Don't be surprised if there's a second nuclear test coming. On February 4, South Korean intelligence officials noticed that there was activity at a second underground nuclear facility — a second tunnel. Those fears are still very real. "North Korea may conduct an additional nuclear test and launch a long-range missile if the United Nations moves to penalize it for its third nuclear test," reports South Korea's Yonhap news agency. And guess what? The U.N. wants to penalize them. Countries like Germany are urging the U.N. to mull more sanctions — not that they seem to have worked in stopping the advance of the nuclear program (here's our take on the diplomacy dilemma) — and North Korea's biggest ally, China, has told them to stand down. "We strongly urge North Korea to abide by its non-nuclear commitment and not to take any further actions that would worsen the situation", China said in a statement picked up by Reuters. And South Korea's Yonhap news agency reports that North Korea has already suggested it would try additional nuke tests if the U.S. reacts with hostility.
• President Obama has denounced the test. "The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community. The United States will also continue to take steps necessary to defend ourselves and our allies," President Obama said in a statement. We're not sure how that rates on North Korea's hostility meter, and if that urging of sanctions will spur another North Korean nuclear test, but NBC News reports that Obama plans to discuss the developments in North Korea in his State of the Union address tonight.

• North Korea hates being threatened. The U.N. will have an emergency Security Council meeting at 9 a.m. to discuss the nuclear test, but even before then, North Korean diplomats are, well, stretching the term "diplomat." Reuters's Andrew Binet reports that one diplomat told the U.N. disarmament forum that North Korea "will never bow to any resolutions" and that South Korea should tell the United States to halt its "hostile policy."
• Obviously, Kim Jong-un's regime thinks this is a glorious day. "The nuclear test will greatly encourage the army and people of the DPRK in their efforts to build a thriving nation with the same spirit and mettle as displayed in conquering space," reads the report from KCNA. (There might be something lost in translation, but North Koreans haven't exactly conquered space.) The report goes on to read that the test will "offer an important occasion in ensuring peace and stability in the Korean Peninsula and the region." Meanwhile, the world very much begs to differ.

This story is still developing, so check back for updates. You can also follow local analysts, North Korea watchers, and news reports for the latest reports overnight.

Original Post: Just hours after reports suggested that North Korea had abandoned its plan to detonate a nuclear device, the U.S. Geological Survey reported a 4.9-magnitude earthquake north of the 38th parallel on Tuesday. The Korean peninsula, by the way, is not prone to earthquakes. Not natural ones, anyways. The United Nations called it an "unusual seismic event" — at first.

Within minutes, a United Nations Security Council diplomat said that there had been a nuclear test in North Korea. And within an hour of that, the AP reported that South Korea "suspects" a nuclear test as well, while Reuters reported that South Korea was "on alert" for additional tests or missile launches. (Analysts focusing on Northeast Asia and North Korea watchers on social media immediately began suspecting a second underground tunnel could mean that a follow-up test might be forthcoming.) South Korea called an emergency meeting of the U.N. Security Council for 9 a.m. Eastern on Tuesday morning, an hour ahead of a scheduled meeting. U.S. officials told CNN they were working to confirm the reports. Japan's Kyodo news service said the Japanese defense ministry was scrambling aircraft to look at radiation effects, though a spokesperson for the government said they were unlikely to spread, and Japanese Prime Minister Shinzo Abe threatened new sanctions. Not that sanctions have stopped the acceleration of the reclusive but provocative state's nuclear and missile programs.

The U.S.G.S. coordinates show that the "earthquake" was centered at the end of a nuclear test road — quite literally — in a location not exactly designed to disguise:

The Comprehensive Nuclear Test Ban Treaty Organization said in a statement that the seismic event "shows explosion-like characteristics and its location is roughly congruent with the 2006 and 2009 DPRK nuclear tests." The South Korean defense ministry claimed the test yield estimated between 6 and 7 kilotons — the atom bomb the U.S. dropped on Hiroshima was estimated at 16.

After all its threats, North Korea had just said that it would continue to test "powerful long-range rockets" but failed to mention the nuclear test talk that stirred the ire of the rest of the world just a few days before. The earthquake cast that all into doubt. Over the course of the past decade, North Korea's been on a collision course with nuclear armament, against the world's wishes, and if reports prove true, Tuesday's detonation would be the country's third underground nuclear test and an obvious act of defiance as the first under its new leader, Kim Jong-un. The test came just before his father's birthday, when propaganda campaigns and international signs of showboating are typical — if not this scary. Whereas the previous two tests used reprocessed plutonium, The New York Times reports that "American officials will also be looking for signs of whether the North, for the first time, conducted a test of a uranium weapon, based on a uranium enrichment capability it has been pursuing for a decade."

The apparent test comes on the day of President Obama's State of the Union, in which he had planned to directly address the drawdown of worldwide nuclear arsenals as his national security adviser prepared to head to Russia to begin talks on a new kind of Start treaty. But U.S. officials appeared to have expected the a "test could come at any moment" and appear to have warned at least Japanese officials.

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Adam Clark Estes
Alexander Abad-Santos
Matt Sullivan