According to intelligence reports, North Korea is preparing its third test of a nuclear device. The test would take place underground. In such tests, a nuclear device is placed into a drilled tunnel about 800 meters below the surface. The hole is then plugged with gravel and other materials to contain the explosion and fallout. The nuclear explosion vaporizes subterranean rock and sends seismic shockwaves through the ground. An underground chamber is created, which collapses and causes a crater on the surface.
Source: BBC

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