The Democratic People's Republic of Korea (North Korea) conducted its sixth (and most recent to date) nuclear test on 3 September 2017, stating it had tested a thermonuclear weapon (hydrogen bomb). The United States Geological Survey reported an earthquake of 6.3-magnitude not far from North Korea's Punggye-ri nuclear test site. South Korean authorities said the earthquake seemed to be artificial, consistent with an underground nuclear test. The USGS, as well as China Earthquake Networks Center, reported that the initial event was followed by a second, smaller, earthquake at the site, several minutes later, which was characterized as a collapse of the cavity formed by the initial detonation.