Simply, the region below the surface of a planet or the diametric opposite surface of a planet - the antipodes. 'Underworld' is also commonly used to refer to the criminal element of a society - see the American Underworld Dictionary.
A common dimension in most of the world's religions and belief systems, the underworld generally refers to the land of the dead - the place where dead souls come to rest.
Inevitably, we have one or two gods assigned to this realm, presumably keeping an eye on the corpses and handling two major responsibilities:
- Order The Greeks assigned Hades to divide up their land of the dead into two regions:
- Punishment - check out 'Seasons of Mist' by Neil Gaiman or 'Satan' by God.
Paranoid as humans are, they generally have the entrance to the assorted underworlds guarded by a fearsome beast of some sort (see Cerberus if you're into the classics or Garm if you like snow) the task of which is more about keeping you out than keeping the dead in.
The Greek underworld - probably the most popular, but, again, all the voters are dead - has five rivers:
- Acheron (river of woe, the Phlegethon and the Cocytus flow into the Acheron)
- Lethe (river of forgetfulness: the new dead drank from this river to forget the pain of their earthly lives so they could be free in Elysium)
- Phlegethon (a river of fire)
- Styx (the entrance to the underworld, across which souls of the dead were ferried by Charon)
AMJ graces us with a nice list of local underworlds below - also consider:
Finally, "Underworld" is a 1997 novel by Don Delillo. It's a story about "trash, nuclear holocaust, and mass consciousness" ... or so nexxus tells me.