An enormous labyrinth, stretching for miles in all directions.... A three dimensional maze of clean-cut marble. The passages are perfectly square, and vast; tall enough for a dragon to walk upright.
The creators, if there were any, paid no heed to verticality; hallways turn towards up and down just as often as east and west.
The labyrinth attracts many people: adventurers, certain that it must guard some treasure; hobbyists, who delight in the puzzle; hermits, eager to find a quiet dead-end to call their own.
Towns have popped up inside, whole cities even, of people who got too deep in to find their way out, and banded together to settle down. Of course, the largest city is at the entrance, serving as both a gateway for arrivals, and a line to the outside for those within.
The most common currency between explorers is the trading of maps. Sections one has explored and charted are swapped, copied, and bartered with others. Each group tends to keep their own maps and records, as much for secrecy as for the fun of the puzzle.
Of course, for those more interested in assembling complete information recorded for posterity, the Cartographer's Guild coordinates the Great Expedition, the largest single cooperative exploration effort in the labyrinth, for the purpose of assembling a complete and accurate map of the entire labyrinth.
