North Korea's Latest Nuclear Test: the Facts

North Korea conducted its fifth nuclear test Friday. Here's what we know so far.

North Korea conducted a fifth nuclear test on Friday, an underground blast that Seoul quickly labelled its "most powerful to date."

Here are some key questions around the blast and the isolated state's nuclear program.

What do we know so far?

Seismologists detected a powerful artificial earthquake at 0030 GMT Friday, which they said was centered around Punggye-ri, North Korea's nuclear test site.

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South Korea says it believes the quake was caused by the testing of a nuclear device, with a yield of 10 kilotons. That would make it the most powerful of Pyongyang's five nuclear tests to date.

Why have they carried out another test?

The North Korean leadership says a credible nuclear deterrent is critical to the nation's survival, claiming it is under constant threat from an aggressive United States.

Although it has regularly threatened neighboring South Korea, its main priority is developing a effective strike threat against the U.S. mainland.

It has always insisted it will continue testing, ignoring global condemnation and toughened UN sanctions.

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Its first nuclear test was in 2006. That was followed by one in 2009 and another in 2013. The most recent test was in January this year.

Experts say the tests are likely aimed at refining designs and reliability as well as increasing yield.

Outside monitors will analyze the yield from Friday's test to try to determine whether it signals any fresh breakthrough.

How advanced is North Korea's weapons program?

The four confirmed tests so far have resulted in artificial quakes of increasing size. Friday's quake followed that pattern, registering 5.3 magnitude.

Seoul said the 10-kiloton yield was the "most powerful to date."

The bomb that destroyed Hiroshima had a yield of 15 kilotons, and the most powerful nuclear test ever was a Russian blast in 1961 with an estimated yield of 50,000 kilotons.

Pyongyang claims its January test was of a miniaturized hydrogen bomb, which has the potential to be far more powerful than other nuclear devices.

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However, scientists say the six-kiloton yield was far too low for a thermonuclear device.

But experts caution Pyongyang is clearly making progress -- regardless of whether or not it has mastered hydrogen bomb technology.

"With each test they can learn a lot," atomic scientist Siegfried Hecker said in January.

The claim of miniaturization is a significant worry for the international community, especially in the light of an apparently stepped-up missile testing program over recent months and years.

If Pyongyang can make a nuclear device small enough to fit on a warhead, and can bolster the range and accuracy of its missiles, it might one day achieve its oft-stated aim of hitting U.S. targets.

How will the international community react?

Condemnation from the United States and its allies will likely be swift and sharp, and will almost certainly include calls for action from the United Nations Security Council.

However, the world will be watching most keenly for what China does.

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Beijing has been North Korea's main diplomatic supporter and protector for decades, shielding its errant ally from harsh international action.

But its patience is running thin. It strongly opposes Pyongyang's nuclear program, which it sees as a source of instability.

And China's seeming inability to rein in Kim Jong-Un is increasingly embarrassing for Beijing: the North's latest missile tests this week came as China was hosting a G20 summit.

However, Beijing's problem is that while it wants Pyongyang to stop causing trouble, it is desperate to avoid anything that might imperil the stability of the regime.

Its nightmare scenario is that if the regime collapses, millions of hungry North Koreans might flood over its border -- and the U.S.-allied South would take over, meaning American troops could be stationed right on the Chinese border.

SEE PHOTOS: A VILLAGE MADE OF BOMBS

style="text-align: left;"> War leaves behind scarred earth and communities left to pick up the pieces, trying to rebuild the life they once knew. And in some cases, using the remnants of war is part of the process.

style="text-align: left;"> Photographer Mark Watson took the photo above during a bike tour from Asia to Indonesia that lasted nine months. "I became curious about the secret war legacy that was here," Watson said.

style="text-align: left;"> You can see more of his story in a new episode of This Happened Here, from Discovery's Seeker network.

style="text-align: left;">"This photo is of local canoes that had been fabricated from war scrap," Watson said. "I spotted them as we cycled over a bridge above and decided to go down and investigate. I recognized them to have been the scattered long-range fuel tanks from American bombers."

World War II Warship Recovered: Photos

style="text-align: left;">"We started to see war scrap everywhere: shrapnel, bomb casings, cylinders from cluster bombs, and the fuel canisters."

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style="text-align: left;">"Scrap from such widespread bombing has been utilized in people's homes and villages," Watson said, "for everything from house foundations to planter boxes to buckets, cups and cowbells."

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style="text-align: left;"> "(The bombs have) also created a scrap metal industry which is now shrinking, but has been responsible for many deaths as poor villages recover and exchange this dangerous resource for money," Watson said. Today there are a number of NGOs working to clear Laos of these mines and bombs. But at the current rate it could take more than 100 years to clear them all. In the meantime, people here are going to have to continue living alongside these deadly relics of war.

style="text-align: left;"> You can see more of Mark Watson's photos here .