The Boom Heard ‘Round the World

Could North Korea have done more to hide its nuclear test?

A nuclear test explosion at Mururoa, Tuamotu Archipelago, French Polynesia, southern Pacific, circa 1971.

A nuclear test explosion at Mururoa, Tuamotu Archipelago, French Polynesia, southern Pacific, circa 1971.

Photo by Michel Baret/Gamma-Rapho/Getty Images

North Korea claimed to have carried out its third nuclear test on Tuesday, after a 4.9-magnitude earthquake was detected near the country’s known underground nuclear test site. North Korea, which the U.N. has banned from conducting such tests, apparently didn’t try too hard to keep the test a secret. Is there any way to hide a nuclear explosion?

Only if you dig a really huge hole. Even then, you’d be limited to testing a pretty small bomb. Due to the wide variety of detection methods used by groups like the Comprehensive Nuclear-Test-Ban Treaty Organization, there’s no sure way to detonate a large nuclear device without practically everyone knowing about it. Perhaps the best bet would be to hollow out a large, round cavern deep underground and blow up a small device at the center of it. Because such a bomb would have a less immediate impact on the rock around it—most of its power would be used up on the surrounding air, in a process called “decoupling”—its seismic impact could be of a small enough magnitude to go undetected. However, because of the difficulty of building such a large hole without drawing attention, it’s unlikely that any country would be able to hide an explosion of more than 1 or 2 kilotons. For comparison, the bombs dropped on Hiroshima and Nagasaki yielded explosions of 15 and 22 kilotons, respectively.

Decoupling has been accomplished on a few rare occasions before. In 1966 the U.S. detonated a 0.38-kiloton explosion. In 1976, the Soviet Union partially decoupled an 8-to-10-kiloton explosion in a 243-foot-diameter salt dome buried almost 3,000 feet below Kazakhstan. However, the explosion was recorded by seismic instruments as far away as Canada. Both cavities had been hollowed out by previous nuclear explosions several times larger, at 5.3 kilotons and 64 kilotons, respectively.

Another theoretical concern is that someone could hide a nuclear explosion by concealing it within an even bigger explosion, such as a large chemical explosion in a mine, in a hypothetical scenario known as “mine masking.” However, such a technique is difficult in practice for several reasons. Perhaps the most significant is that such large explosions, especially ones that are instantaneous in a manner similar to nuclear bombs, are extremely rare, and so just about any explosion of this size would attract some suspicion. This would, once again, mean that the device would have to be relatively small. Along the same lines, the proposed method of “hiding signals of an explosion in that of an earthquake” is impractical, both because of the difficulty of synchronizing the explosion with the time and location of an earthquake, and because the explosion’s seismic signature could still be filtered out.

Detonating a large nuclear device either underwater or high up in the sky would be even more difficult to hide. To detect explosions underwater, the CTBTO has 11 hydroacoustic stations in eight countries, which listen for sound waves travelling through the oceans. To detect explosions in the air, it has 80 radionuclide stations in 27 countries, each of which can detect radioactive particles blown by the wind from nuclear test sites miles away. Even explosions deep underground can release radioactive noble gases to the surface, which can be detected by 40 of the radionuclide stations. And these aren’t the only ways explosions can be detected from far away. Most explosions could be seen on satellites and could cause any nearby gamma ray and radio telescopes to light up with the sudden activity. They also cause a large electromagnetic pulse, which can cause radio interference, power surges, and can even fry some electronic devices. The CTBTO also has 60 stations sensitive to low-frequency sound waves (infrasound) given off by large explosions. Underground tests can even be visible to the naked eye, as they can cause the ground to settle in above them, leaving a large implosion crater.

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Explainer thanks Howard Hall and Steven Skutnik of the University of Tennessee.