The blast was picked up by seismic stations all over the world, and it was big. The Comprehensive Test Ban Treaty Organization, which monitors the globe for nuclear tests, said that its monitoring system had gone off-scale . The U.S. Geological Survey recorded a 6.3-magnitude earthquake , which was human-made. That's far larger than the seismic signature from the North's last test, conducted roughly a year ago . Here's what you need to know. This was probably not an "ordinary" nuclear test North Korea's previous nuclear tests have been in the tens of kilotons range. That corresponds roughly to a weapon the size of the ones used on Hiroshima and Nagasaki at the end of World War II. It's believed that the North's earlier tests were of nuclear weapons that use uranium or plutonium (or both) for their explosive yield. This time, the North claims to have mastered a far more powerful hydrogen weapon . Some early estimates are putting this test in the hundreds of kiloton range. "That's very
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