The Largest Underground Nuclear Blast Ever
The largest underground blast ever and largest US blast outside the South Pacific Operation Cannikin Alaska 1971 The Primary reason was to test the new warhead for the Spartan missle. Because of the vaporised underground rock, a crater formed within 2 days that was 55 feet deep and 1and a half miles wide.

I wonder if the DPRK has seen this? The US still rocks!