By John Sudworth
BBC News, Seoul
So, North Korea has carried out what it had been threatening to do for weeks, another atomic test.
Some South Koreans were angered by the North's test
The announcement broadcast on its state media said the detonation, which rattled the Korean peninsula with the magnitude of an earthquake measuring 4.5, was "in defence of the nation, the country and socialism."
It was South Korea that was first alerted to news.
Seismologists at the National Geographic Centre picked up the tremors 350 km (220 miles) away on the other side of the border.
The South Korean government immediately called an emergency cabinet meeting, although there was little sign of real public concern, apart - perhaps - from a small group of demonstrators who took to the streets to burn models of North Korean missiles.
The explosion is believed to have taken place in North Korea's north-east region, close to the site of its first nuclear test, in 2006.
This one was more powerful.
The blast is said to have made the ground tremble as far away as the Chinese border.
It seems, then, that reclusive North Korea, with a one million strong army, has dramatically increased its nuclear capability.
And not surprisingly, it has come in for strong international condemnation, with political leaders in Seoul, Tokyo and Washington declaring it an illegal act, a breach of an existing UN resolution.
Even Moscow and Beijing pointed fingers at Pyongyang, with China expressing its "resolute opposition" to the test.
So why has North Korea once again invited pariah status and carried out this nuclear test?
It had already announced that it was preparing for the test as a response to the international criticism of the rocket launch it carried out last month, although it gave little away about the exact timing.
It is hard to tell what North Koreans really think
North Korea's leaders claimed that the rocket, fired from its east coast launch base, was designed to put a communications satellite into orbit, and appeared infuriated by suggestions that the launch was, in fact, a cover for a long-range missile test.
Condemnation by the UN Security Council, it said, was the final straw.
North Korea's logic ran like this: If the world refused to play fair and recognise its rights, why should the North honour its obligations?
On 14 April, North Korea announced it was quitting the long-running nuclear negotiations, known as Six Party Talks, for good.
But there are many observers, conservatives in Seoul and Washington for example, who have long doubted whether North Korea every really intended to give up its nuclear weapons programme.
And, they have been suggesting, it has simply decided that, for now at least, it may have won all the concessions it can it terms of aid and trade.
Uncertainty over future
North Korea may feel its interests are better served by enhancing its strategic capabilities, possibly to win a bigger payout further down the line.
Some analysts say this nuclear test could be an attempt to shore up the legitimacy of a potentially weakened leader in the eyes of a domestic audience
But there is another consideration.
The North Korean leader, Kim Jong-il, is rumoured to be in poor health and has rarely been seen in public in recent months.
With no clear successor, there is deep uncertainty about the future direction of the country.
Some analysts say that this nuclear test could be an attempt to shore up the legitimacy of a potentially weakened leader in the eyes of a domestic audience.
Of course, it is impossible to know what North Koreans really think.
It is likely that there may be some genuine pride in each announcement it makes concerning its military prowess.
But if there weren't such stringent restrictions on freedom of speech, maybe some North Koreans might wonder aloud about how it is that one of the world's most impoverished regimes has just taken yet another defiant step towards nuclear weapons status.
NORTH KOREA'S SUSPECTED NUCLEAR TEST SITE
Nuclear test on 25 May 2009 is thought to have been carried out at same site as October 2006 test