In the mid-morning, local time, of January 6, an underground blast in a remote corner of North Korea sent seismic waves worldwide, leaving clear recordings on thousands of seismometers. This is the fourth detonation that has been detected at the North Korean nuclear test facility. Prior events occurred in 2006, 2009 and 2013. The yields of North Korea’s previous explosions grew in sequence, by factors of 3 to 5 in size, until the 2013 detonation achieved the size expected for a first-generation plutonium fission device. The 2006 and 2009 detonations were widely regarded as failures to achieve North Korea’s technical goals for explosive yield. The 2013 detonation was large enough to conclude that North Korea had built a nuclear device equal to that built by the United States in 1945, by the Soviet Union in 1949, and by several regional nuclear powers since.