From a 2002 Korean news story:
Kim Hyong-jik County, Yanggang Province, in which a large-scale missile base was learned to have been completed in the 1990s

From a story today:
It was reported that there was a massive explosion Thursday around the town of Yongjo-ri, Kim Hyong-jik County, Ryanggang Province. U.S. Department of State, sources familiar with North Korea and the Korean government all confirmed the explosion.

Now, people are claiming that this is a "forest fire", but that doesn't really seem to be consistent with reports like this:
A high-ranking government official said Sunday, ¡°It is true that a large mushroom cloud about 3.5 to 4 km in diameter was observed by a satellite at around 11:00 a.m. Thursday. It was not a nuclear test, but the explosion seemed to be three times bigger than the one that took place during the Ryongchon Station accident

Of course, a lot of people question whether or not what happened at Ryongchon was an "accident".

And then, of course, we've also got trains exploding in Iran.

Interestingly:

On April 22, while trains loaded with chemical component fertilizer and tank cars containing oil were being repaired at Ryongchon Station in North Pyongan Province, they came in contact with an electrical wire through carelessness, setting off a massive explosion. (April 24, Korean Central News Agency, Pyongyang Broadcasting Station, Korea Central Television Station)

Last Wednesday, a string of 51 wagons loaded with chemicals, fuel and fertiliser derailed and exploded at Khayyam station near Neyshabour, some 75 kilometres (50 miles) from the northeastern city of Mashhad.

Apparently both the North Koreans and the Iranians like to turn their trains into rail-bound bombs.

My suspicion is that what we actually saw was some sort of attack on North Korea's major missile base.