When one refers to the Haven Mines, one is rarely referring to the few lesser mines still in operation in Haven, a castle-town a few miles from the eastern border of the Silkwood. No, "Haven Mines" is a particular place -- a deep and twisting cave complex once mined for its abundance of iron. Peak excavation was long ago, and the mines grew steadily more dangerous to work. When there were still miners, they complained of being haunted by sleepless spirits of their fallen fellows, who would allow them no rest. The legend of Flint Slegemiser was, for a time, the worst thing about the mines (aside from the back-breaking labor, rock dust, and creatures in the dark).
Bands of ogres then discovered the entrance at the edge of the forest, and squabbled and feasted in the mines, apparently unbothered by any ghosts in their new lair. Then, of course, came the gnolls after them, who were so alarmingly vicious and cunning that Haven wished it once more had brutish ogres for neighbors.
The Lord of Haven ever eyed the distant mines with dissatisfaction, sure that a wealth of iron still awaited the bold -- or, perhaps other, more glittering riches, deeper still. Many an adventurer brought monstrous heads to his castle in attempts to reclaim them, but all that these bounties earned Haven were increasingly cunning enemies and the sight of adventurers packing home their found-iron from Lord Rodor's abandoned mine.
Eventually, some series of events (involving adventurers, as usual) led to the flooding of the mines and collapse of the entrance (if not more) -- no doubt to the secret relief of Haven's citizenry.