Depending on your point of view, the rationale behind the existence of this dungeon may be the most important factor of dungeon design or the least important. At any rate, it's the final table of this dungeoneering tour. Or you could make it the first.
This is the thirtieth table of Your Dungeon Tour Table.
Roll 1d301. It is a standard dungeon haunted by those who perished in it.
2. It is a standard dungeon currently operating as such. What are you doing here?
3. It is an abandoned dungeon infested with monsters who burrowed from below.
4. It is an abandoned dungeon occupied by monstrous invaders.
5. It is the product of a mad wizard's mind.
6. It is a network of tunnels belonging to a secret cult that delved too deep.
7. It is an abandoned mine that accidentally struck the subterranean domain of something inimical to surface dwellers.
8. It is the practical joke of a godling.
9. It is a gigantic spacecraft that crashlanded.
10. It is an amusement park of the ancients.
11. It is a network of tunnels being used by several criminal factions.
12. It is a shopping mall of the ancients.
13. It is a former menagerie of diverse monsters who now roam freely.
14. It is a natural history museum of the ancients.
15. It is an obstacle course for assassins.
16. It is an isolated utopian/dystopian complex built to survive an apocalypse.
17. It is a subterranean shrine to evil gods.
18. It is a metaphor for our relationship with Death. (Metaphysics optional.)
19. It is a market connecting traders of the underearth to nocturnal traders of the surface.
20. It is an illusory environment created by the minds of bulging-headed aliens.
21. It is a nightmare dimension constructed by shadow demons.
22. It is the virtual world of a fantasy video game.
23. It is a playset for juvenile gods or juvenile godlike aliens.
24. It is an underground city forgotten by the city above it.
25. It is a weird manufactory staffed by elves, dwarves, or cyclopes.
26. It is the ancient labyrinth of a lost civilization.
27. It is a viable subterrestrial ecosystem.
28. It is a twisted behavioral science experiment conducted by powerful beings.
29. It is a game show with fabulous prizes!
30. It is an incomprehensible game played by the gods.
N.B. The result of this table may or may not conflict with the result of the Dungeon Ambience table, in which case you may choose one to override the other, roll again, or mash them together.