There's an underground river on that map level, that looks as though it enters a cave. With the extreme challenge the coal mines and alhoons lair can present on the higher spawns, I had thought it would be a good place for a mid level dungeon, possibly a variable one, based on the creatures that spawn onto that map. The challenge would need to be slightly higher than the map outside, and I would suggest keeping the loot drops in it reasonably low still, but it would give lower level characters, especially dwarven ones, a chance to get a bit more sense of accomplishment out of a trip there.
Screenie for reference of the spot I mean:
