Nuke test in North Korea?

Well, not one big enough to detect from south korea's seismic monitor..
EDIT:
USGS picked it up.

More from Globalsecurity
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The named place that is most nearly proximate to the possible nuclear underground test site is Chik-tong, a small populated place located at 41°16'00"N 129°06'00"E. The suspect site is quite isolated, and is to be found several kilometers away from Chik-tong, and several kilometers removed from Mount Mantap.

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Checking google earth the test site is 5 kilometres from the epicenter reported by the USGS..
( Image behind cut )

Google maps link. You can see there's a road there..