Should North Korea follow through on its threat to detonate a hydrogen bomb over the Pacific Ocean, it would be the first atmospheric nuclear explosion in almost four decades.

More than 500 nuclear tests have been conducted above ground around the world, according to the Comprehensive Nuclear Test Ban Treaty Organization, but none since 1980. All of North Korea’s six nuclear tests have been held underground, the first in 2006 and the most recent on Sept. 3 this year.

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