Well, that's what seems to be indicated by this AP Article.

SEOUL, South Korea (AP) -- A large explosion rocked the northern part of North Korea, sending a huge mushroom cloud into the air on an important anniversary of the communist regime, South Korea's Yonhap news agency reported Sunday.

Citing an unidentified source in Beijing, Yonhap said the explosion happened on Thursday in Yanggang province near the border with China. The damage and crater left by the explosion in Kim Hyong Jik county was big enough to be noticed by a satellite, the source said.

"We understand that a mushroom-shaped cloud about 2.2 miles to 2.5 miles in diameter was monitored during the explosion," Yonhap quoted an unidentified diplomatic source in Seoul as saying.

It goes without saying that this is alarming (if as-yet unconfirmed). Of course the real problem is that there isn't really much that can be done, aside from detering the North Koreans with the threat of force. After all, if they've tested, they've (as many suspected) probably had nukes for years.

The last window to stop them, I believe history will record, closed in 1994 or so.

More to come.