The key to something like that is to offer situations that encourage entering or exploring the dungeon as a natural desire of the PCs. Bandits using the upper levels as a base. A zombie-fungus plague showing up in town, and PCs gradually discovering the source seems to come from these caves (that link up to the dungeon). An NPC adventurer party showing up and hauling fat loot up, and being lauded by the townsfolk (in my experience, this works exceptionally well, because the players look at it as upstart NPCs stealing their fortune and glory). The party needing to find some artifact or item that is rumored to be deep in the dungeon. A hated enemy that flees into the dungeon. Et cetera.robertsconley wrote:The time you probably see this happening the most is when the DM wants to run an adventure path or something big like Castle Whiterock or The world's largest Dungeon.
(Sound obvious, I guess, but I'm surprised at how often this kind of thing seems to get overlooked. And if they don't "take the bait," no big deal -- run with what they want to do. I typically try to set up multiple rumors or situations.)
(EDIT - The thing about a big dungeon is that it's a sort of "adventure gravity well" in its own right. That is, just by its presence in the setting it will naturally tend to come into play, even when play starts off on a seeming tangent that has nothing to do with it.)