Every time North Korea has tested an atomic weapon underground, the explosion has released a wave of seismic energy that moves through the ground like sound moves through the air. That's why seismic monitoring stations across the globe can essentially hear the explosions and record them in precise detail.
In the video below, Zhigang Peng, a geophysicist at the Georgia Institute of Technology, has converted seismic data from each of North Korea's four nuclear tests — including the one that occurred Tuesday — into audio.
You'll hear that they all sound remarkably similar, which made it easy for geophysicists like Peng to assert that yesterday's seismic activity out of North Korea wasn't a natural earthquake but an underground nuclear test.
To make the track, Peng made the frequency of the data 100 times faster so it falls within the range we can hear. "This is similar to how we play the fast-forward cassette player to hear higher pitch in the old days," he writes me in an email. They're all sped up by the same amount, so it's easy to make comparisons. The data was collected from a seismological station in Mudanjiang, China, which is about 100 miles from the North Korea border.
Of the four tests, he says, "They all sound very similar, indicating that they are similar types and from similar regions."