In September 2017 North Korea tested its largest nuclear bomb yet. It was 10 times the blast strength of any of the five previous underground detonations (map). How do we know? A global network of more than 300 earthquake-monitoring stations stands sentry. After an explosion, seismometers pick up two types of shock waves within minutes and alert intelligence officers. Scientists learn even more afterward. Since the last blast, they have combined the seismic signals with satellite images and other data to pinpoint more details, recently published, such as the precise location and bomb size (chart). Should North Korea—or any other nation—try another bomb, the world will know.

Credit: Katie Peek; Sources:“The Coupled Location/Depth/Yield Problem for North Korea’s Declared Nuclear Tests,” by Michael E. Pasyanos and Stephen C. Myers, in Seismological Research Letters. Published online August 8, 2018 (bomb sizes and blast depths); “Absolute Locations of the North Korean Nuclear Tests Based on Differential Seismic Arrival Times and Insar,” by Stephen C. Myers et al., in Seismological Research Letters. Published online August 15, 2018 (test locations); www.Norsar.No (seismograms); “The Punggye-ri Nuclear Test Site: A Test Tunnel Tutorial,” 38 North, May 23, 2018 www.38north.Org/2018/05/Punggyetunnel052318 (tunnel locations); U.S. Geological Survey (earthquake magnitudes