Interesting remark.
I did a FReep search to see if any new info was posted about that explosion. I remembered there was one report of a crater, and another that said there were 2 explosions, but I saw no follow ups. While the presence of a crater would seemingly point towards a surface explosion, instead of tunneling (but not necessarily), explaining a pair of explosions is a bit tougher (could point to an accident where there could have been a secondary explosion, or, could point to a construction project of some sort. Take your pick)
But one thing did stick out in my mind as I looked at the reports made at the time: geography.
If one just looks at a simple surface map, like the one included in one of the articles, that explosion took place at the point in North Korea that is furthest from the water, where one would expect the US Navy to be, and the furthest from S. Korea.
I know that strictly from geographical viewpoint, I would want an important strategic site placed there, to help protect it from easy attack, relatively speaking. By maximizing the distance an enemy is going to have to cover, the chances of detecting an attack, and possibly defending against it, increase.
One thing is for certain: the North Koreans have had ample time and practice since the Korean War to have become experts at tunneling...