We can think of culture, the product of civilisation, as a living thing. Then consider the means of culture to seek it's own survival.
Those means will include a freight-train sized centipede.
The Civilopede is a scavenger/predator. When the cities of the Underdark burn and fall, the Civilopede arrives to hunt through the wreckage, discover and preserve the artefacts and records, and ruthlessly hunt the survivors.
A few librarians and archivists are spared, they enter into a symbiotic relationship with the Civilopede, becoming its back-riding teamster-curators.
The nerdy remnants of every race swarm on the creatures hide, arranging and re-arranging the endless piles of art, artefacts, museum pieces, records, writing and treasure. Anything you could find in a gallery, museum, archive or library will be stored there.
The only place in the Underdark that culture could be preserved in this way is by something big, scary, violent and dangerous enough to keep it safe.
The Civilopede never stops moving in its endless perambulations beneath the earth. It can be mounted, with great danger and risk. The curators will turn against any invaders unless they bring or produce culture, so boarders can exist safely, but trapped in a kind of endless salon. Decadent aristocrats of civilised races sometimes try to board for exactly this reason.