In the Demiurge’s memory theatre
…deep in the uncharted wilderness the players break through a cruft of goblin nests to discover an enormous, multi-layered gallery filled with tableaux – statues locked in epic combat; miniature cities under attack by squadrons of flying carpets; glittering rooms peopled with uniformed porcelain figures grasping alien devices; cathedral caverns filled with colossal stone elephants and whales. Everything pointing to everything else like some gigantic, ancient explanatorium. Some of the exhibits are made from plain stone or glass, others from huge blocks of malachite or chalcedony, some crafted from gold and gems.
Of course, this buried museum, or art-house, or whatever it is, isn’t uninhabited, it’s just incidentally inhabited with an awful lot of inanimate stuff. And you could prize the gems out and run away and make a fortune. But the real money would be in pulling out whole statues – and that would take manpower, effort, time. And if you did, then the real value would be lost – as some sage will eventually tell you – because it’s only by having all the figures in their original setting that you can read them for their deep, revelatory meanings. So then you should build a town at the entrance to this wonder, and guide sages through it so they can plumb its secrets. And you should replace what you looted, so they can read whatever you unwittingly destroyed.
But it’s only by having all the original parts in place that you can make the ritual work, to bring it all to life and opens a portal to the planes/save the world from boredom/remake the PCs as demigods/lift the curse of endless background.
…so the megadungeon campaign, if the players bite, eventually runs in reverse. First they loot it and sell the bits, then they try to make new bits to replace what they stole, and finally they have to run all over the world tracking down all that stuff they previously hoiked to put it back in its proper place – they become dungeon makers, rather than dungeon despoilers.
And that’s how you get them to go into the Castle of the Mad Archmage, which they otherwise would absolutely never do. To get that last wind-up box.