Shatter cones are surface features with distinctive wavy patterns that are known to be created only by the tremendous force of a meteorite impact or an underground nuclear explosion.

MSN: Massive meteorite crater found in Canadian Arctic

Pyongyang is angry about ongoing U.S.-South Korean military drills and about new U.N. sanctions, issued over the North's December long-range rocket launch, which the U.N. called a cover for a banned missile test, and its third underground nuclear explosion, conducted Feb. 12.

NPR: NKorea Criticizes SKorea Prez's 'Swish Of Skirt'

Nuclear test monitors in Vienna say the underground explosion had double the force of the last test, in 2009, despite the use of a device said by the North to be smaller.

BBC: Obama: Nuclear test 'isolates North Korea further'

Nuclear test monitors based in Vienna say the underground explosion had double the force of the last test, in 2009, despite the use of a device said by the North to be smaller.

BBC: US seeks UN sanctions targeting North Korea diplomats