Hidden somewhere on the referee's map are the treasures, the gold, the magic ring, the laser cannon... Perhaps an ancient magic race created a city or a temple which subsequent ages have buried. The original inhabitants have all died or migrated off, although their magic spells remain in full force. Bands of wandering orcs move into the upper chambers. Giant rats infest the corridors, feeding on the bodies of unlucky adventurers. An evil wizard finds new magic powers in the ancient city, and he sets up his own guards and traps... Dragons and giants loot the surrounding countryside, and over the ages they accumulate fabulous wealth.
-Dr. Holmes, Fantasy Role Playing Games, page 41. Thanks to OD&D Discussion board member aldarron for posting this!