Let's say you're North Korea and you have this nuclear device you really want to test. And let's say you'd rather some of the more sensitive details remain private.
Physicists, geologists, imagery analysts, some of the best militaries in the world, monitoring posts set up by non-proliferation organizations — beating the technology arrayed against you will be no mean feat.
As soon as you detonate your device, seismographs all over the world are going to pick it up. If it has a decent yield, it will look like a moderate earthquake, but not to the experts — who will quickly identify it as a man-made explosion. Military "sniffer" spy planes — probably already in the area because your preparations have been caught by spy satellites — will soon be scouring the air for samples of elements that can be used to analyze your blast.
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But, it turns out, they might not actually find very much.
North Korea has proven over the past 10 years that it can be exceptionally difficult to determine from a properly set up nuclear test some of the most basic details an adversary would want to know.
Here's a look at how they do it, and why they keep it up.
THE TUNNELS OF MOUNT MANTAP
North Korea has conducted five nuclear tests, the first in 2006.
All were conducted in the depths of Mount Mantap, a nondescript granite peak in the remote and heavily forested Hamgyong mountain range about 80 kilometers (50 miles) as the crow flies from Chongjin, the nearest big city. North Korea is the only country in the world that still conducts nuclear weapons tests, so its Punggye-ri site on — or mostly under — Mount Mantap is also the world's only active nuclear testing site.
Its existence is hardly a secret.
It's a favorite target of spy satellites — military and commercial — and North Korea even released a propaganda film depicting it in 2010.
Located about halfway up the 2,205-meter (7,200-foot) mountain, it has three visible main entrances, or portals, into a series of horizontal tunnels stretching a kilometer (about a mile) or more into the mountain. Studies of the tunnel used for the second test, which was conducted in 2009 and featured in the propaganda film, suggest it has the shape of a fish-hook, which is what experts might expect. Pakistan used a similar design.
When a test is conducted, which in the North's case has always been between 8 and 10 in the morning, the nuclear device is placed at the farthest end of the tunnel, which uses the angles and corners of the "hook" section to deflect and absorb as much of the blast as possible. To further optimize absorption, the tunnel, believed to be about 3 meters (yards) wide and 3 meters high, has nine or 10 sharp corners with bulkheads and several dead ends that serve as debris traps.
Most of the blast is intended to be absorbed within the area defined by the first four bulkheads. To prevent ejecta from escaping into the atmosphere — and to further contain the explosion itself — sand, gravel or other materials can be mixed with concrete to plug, or "stem," segments of the tunnels.
It has been estimated the most recent test was conducted below 700 meters (770 yards) of solid mountain.
Concern that another test may be looming has been heightened recently by images from satellites and analysis by experts Joseph Bermudez and Jack Liu on the website 38 North. The latest unclassified imagery, from March 25, suggests communications cables used to initiate a test and collect data may have been laid already. They also detected water being pumped out of one portal, "presumably to keep the tunnel dry for monitoring or communications equipment."
But subterfuge could be at work here as well.
North Korea is aware its site is being watched and activity can be staged to create a false alarm.
KEEPING THE GENIE IN THE BOTTLE
North Korea didn't do an especially good job of obscuring its 2006 test.
Xenon and krypton isotopes — the smoking guns of a nuclear test — were detected in the atmosphere. That made it possible for scientists to conclude Pyongyang had used a plutonium-fueled device. Adding insult to injury, seismic data suggested the test was less than a complete success, more of a fizzle than a bang.
In 2009, North Korea used a new tunnel and no such gases were detected.
In 2013, xenon isotopes were detected — but possibly because North Korea drilled back into the mountain after the test, not from the test itself. Moreover, the samples were collected nearly two months after the fact and were too degraded to answer the biggest question: Did the device use plutonium or highly enriched uranium?
Being able to build a bomb with highly enriched uranium is important for two reasons: The North has only limited supplies of plutonium, so uranium enrichment would allow it to build a bigger stockpile; and uranium enrichment facilities are easier to conceal. It is widely believed the North has tested both.
There are other North Korean claims that experts would like to evaluate. Pyongyang says it tested an H-bomb in January last year and that September detonated its most powerful device to date. It further claims the September test proved it can put a nuclear warhead on a long-range ballistic missile.
David Albright, a physicist and founder of the nonprofit Institute for Science and International Security in Washington D.C., said that without better evidence such as noble gases, such claims cannot be confirmed or denied.
Albright said the lack of signature gas samples shows North Korea has been "surprising in its effectiveness," considering the number of tests it has conducted and the difficulty of preventing leakage not only into the air but into mountainous terrain with natural cracks and flowing, underground water.
WEIGHING THE FALLOUT
There are all kinds of reasons a nuclear test isn't a great idea. Each one North Korea conducts invites more international condemnation, tougher economic sanctions and anger from China, which is its economic lifeline and former Korean War comrade in arms.
But Pyongyang has stated repeatedly that its "nuclear deterrent" is the cornerstone of its defense against what it views as a hostile U.S. Showing off what you can do — especially if you can do it in a manner that suggests even greater capabilities — can be of tremendous benefit from the military perspective.
Though debates persist regarding the true yield of the blasts — another detail that can be hard to pin down with precision — they are believed to have been on an upward trend, with the most recent coming in at somewhere between 10 and 30 kilotons. For comparison, the bomb dropped on Hiroshima had a 20-kiloton yield.
"My guess is that the North is happy to have the world see that it is testing and get an estimate of the yield — at least as long as it is increasing — but likes keeping the world guessing about how advanced its program really is," said David Wright, a senior scientist at the Union of Concerned Scientists.
Albright, meanwhile, said he is particularly concerned the North is trying to develop and master the use of thermonuclear bomb materials such as weapons grade lithium-6 to enhance the power of its arsenal. He also suspects it is looking at new fission designs, such as bombs with composite cores of plutonium and enriched uranium.
The payoff could be smaller, but deadlier, bombs that would be easier to put on long-range missiles and would use less plutonium per weapon.
In other words, expect more testing.