On the international stage, there are few things more frightening to the world at large than the looming possibility of nuclear war. Many nations have the bomb — some with fission-only bombs, others have achieved the deadlier nuclear fusion — but not everyone publicly declares what they have. Some detonate nuclear devices while denying it; others claim to possess fusion bombs when they don’t have the capability. Thanks to a deep understanding of science, the Earth, and how pressure waves travel through it, we don’t need a truthful nation to figure out the real story.

In January of 2016, the North Korean government claimed that they detonated a Hydrogen Bomb, which they promised to use against any aggressors that threatened their country. Even though news outlets showed photographs of mushroom clouds alongside their reporting, those aren’t a part of modern nuclear tests; that was archival footage. The radiation that gets released into the atmosphere is dangerous, and would be a clear violation of the 1996 Comprehensive Nuclear-Test-Ban Treaty. So what nations generally do, if they want to test nuclear weapons, is they do it where no one can detect the radiation: deep underground.

You can detonate a bomb anywhere you like: in the air, underwater in the ocean or sea, or underground. All three of these are detectable in principle, although the energy of the explosion gets “muffled” by whatever medium it travels through.

  • Air, being the least dense, does the worst job of muffling the sound. Thunderstorms, volcanic eruptions, rocket launches and nuclear explosions emit not only the sound waves our ears are sensitive to, but infrasonic (long wavelength, low frequency) waves that — in the case of a nuclear explosion — are so energetic that detectors all over the world would easily know it.
  • Water is denser, and so although sound waves travel faster in the medium of water than they do in air, the energy dissipates more substantially over distance. However, if a nuclear bomb is detonated underwater, the energy released is so great that the pressure waves generated can very easily be picked up by the hydroacoustic detectors many nations have deployed. In addition, there are no aquatic natural phenomena that could be confused with a nuclear explosion.
  • So if a country wants to try and “hide” a nuclear test, their best bet is to conduct the test underground. While the seismic waves generated can be very strong from a nuclear explosion, nature has an even stronger method of seismic wave generation: earthquakes! The only way to tell them apart is to triangulate the exact location, as earthquakes only very, very rarely occur at a depth of 100 meters or less, while nuclear tests (so far) always have occurred just a small distance underground.

To this end, the countries which have verified the Nuclear Test-Ban Treaty have set up seismic stations all over the world to sniff out any nuclear tests that occur. Continue reading.