I saw this in a thread on megadungeons over on the RPGsite, and it's worth reposting:
IMC Law is in the sky, and Chaos is beneath the ground. Near the surface the chaos is weak; the deeper you go, the stronger Chaos grows. Weaker chaos things climb as high as they can, to keep their distance from the stronger predators beneath. Stronger chaos things need to remain in the depths where chaos sustains their power.
The higher you climb, the more perfect the order of the heavens, until you reach the sphere of the stars where everything moves in perfect circles. The light of the sun is the light of law, and it blasts most chaotic things out of existence. So the dungeon entrances are best avoided at night but the villages are, by and large, safe----even though they're often only a day's travel or so from the entrance to a mythic underworld teeming with terrible magical predators.
Early in time, Chaos made a thing that could live on the surface (elves). Law made a thing that could live beneath the ground (dwarfs). Neither was very satisfactory for their makers, and both tend towards neutrality; their makers abandoned them and now they're grouped with humanity.
Anyway, life is chaotic. It teems and seethes. Life can't survive in the heavens, but it constantly crawls and scuttles up from the depths. The dungeon predators eat one another, of course, but their numbers are always being replenished by the raw chaos beneath.
Chaos is linked to greed, so a thing created by chaos often has treasure on it or near it at the moment it appears. Generally a weak creature loses much of its treasure as it climbs towards the surface (in payments to stronger creatures for safe passage).
- -Author P&P (Papers & Paychecks)
I like how this ties Mythic Geography to both cosmic Alignment and demihuman origins, while also providing an explanation for dungeon depth = danger, and why treasure tends to pool in the deeper dungeon levels. It's a fantastic background cosmology for your BX or OD&D game, and upholds the dungeon as an underworld. Great great stuff.