I like dungeons and I'd like to use more of them but I'm having trouble thinking of a way to introduce them into a narrative again.

So far I've used:
Old dungeon under the city holds a secret as to how the city was founded.
Castle was magic air-tightness sealed and sunk into the bottom of a lake to stop people getting the macguffin.
And Over-large fast flowing sewer system, accessible by a party on a raft only this dungeon held a Aboleth and it's servants.

They've all worked quite well, but what I'd also like to try is having a dungeon as an ongoing obstacle. I don't mean that it should take 5 sessions to solve, I mean it should be a reoccurring element of the overall adventure. However once the party start to think "this session we're doing the dungeon" they probably won't find a reason to leave on their own before the dungeon is done.

Share the dungeon concepts you've seen or done and tell me anything special about the role the dungeon played in the story.