There is a parasite called the expanding wurm. Its body parts are used, in safe quantities, to create potions of growth and giant size. However in its natural state, the worm infects creatures to grow many times their own size, becoming more food for the worm's offspring. The worm managed to infect a badger, which tunneled deeper and deeper to make up for its massive bulk until it died. Now its bones and chambers have become a slowly rotting dungeon; deep under the hills.
 Remember your childhood home. The more you think on it, the more you invent and add to the memory. Sometimes, Wizards do this, and sometimes, Wizards go mad. Her memory was placed in the real world; converting everything nearby into a sprawling dreamscape of half remembered truths.
 There was a hammer left over from when the Gods made the world. They left it on a mountain somewhere, forgotten. Then, a goblin got their hands on it, and made a dungeon as if a child's drawing of a house and summer day could manifest as a real thing.
 It's a magical palace, built to harbor it's residents as they traveled through different planes. It accidentally crash landed here. The crew is all dead now, their machines and spells running rampant all on their own.
 Castle of the blinded king. It has no windows, and was built intended to be pitch black. The King who lived their remains so in undeath, his fingers guided along the castle's walls, still trying to rule over a kingdom that no longer exists.
 The treants once had a city too, made of their spare limbs, old friends, failed children. The wood was magic, and all was partially alive, as old trees never really die. The city walks.
 The most ancient of Gods was a collector. Taking everything beautiful and shiny along with it, and creating a great gray-skinned beast with tusks of ivory to carry them. The elephants broke free from the chains of the God's collection they were once made to drag around, and became a creature on its own, but the collection remains there, wheels long rusted stalled.
 The last emperor had an impressive menagerie. Beasts of all sorts and kinds from all over the world were gathered here. One creature, a Ephemeral Bat, was so rare and special that the emperor ventured to get a mate for it. The resulting disaster caused the entire zoo to sink into the ground and be abandoned. Some of the animals have adapted to the darkness, and live in their new little ecosystem down there behind broken bars.
 In the blue bayou there is a massive gator that doesn't die. It just gets older, and bigger, and only the upper half of its body is ever seen, peaking from the water to gobble up boaters, fishermen, rafts, and beasts alike. It's bottom half is buried underneath the depths of the swamp in its massive, bloated tail. Within that fatty tissue lay a collection of all the hard matter its ever ate; gangplanks of wood jut out into the black pit that goes deeper into its digestive system, innocent rafters contend with swamp cannibals alike; warping into mutants and monsters within the monster's body forever. Don't try to damage the walls, else the alligator swallows down a huge amount of water; washing away anyone who fails a saving throw.
 The lust addled minds of the scrimshaw sorcerers bound themselves in the finest silks, dashing madly through mountain canyons with knives outstretched. The grooves they made lead deeper and deeper in, and entire ecosystem now lives in those dark crevices and cracks, and the narrowest points seem almost purposefully carved to make it uncomfortable, but always possible, for a man to pass through with a tight squeeze.
 Portal to another world opens. To prevent the invasion of ours from theirs; a magic barrier was erected. Now that chunk of their world was cut off, separated, and became a dungeon in its own pocket dimension. If the emitter within is destroyed, the other world shard may join with outs, or theirs, or may just be destroyed ending everything within the shard in an instant.
 The master painter of the Guild has created his magnum opus; a painting featuring a dusty hill with a ruined fort in the background, a gentle stream besides, and a curious cloud. His skill is so great however, that he didn't realize he painted the fort in the background to become real, a dungeon with blurred and painted monsters within; you can enter the paining like a doorway. Paint thinner burns here like the strongest acids; so use it against the painted monstrosities.
 It is not a dungeon, but a tower. They tried to build their way up to God. Something else met them, above the clouds.
 Every night, cats go out on the town. But if you follow a drunken cat and are just stealthy enough; you will find its secret path to the city of cats, which is a secret realm behind every city. The city of cats is cast in night, and while the cats themselves are safe, the various creatures of the city don't take kindly to intruders. Ancient treasures of the not!Egyptians are stored here; given as offerings to the cats as living gods.
 In the center of the hottest desert in the world lies a palace of never melting ice behind an unassuming sand dune. Red sand, converted from the air and water of the desert, pours out from the windows and doors, mix with the yellow and white sands of the desert, spreading the influence of the palace's cruel master.
 The Catacomb. Every one of any worth who has died has had their remains brought here; a procedure close to the God of death. Even those who lived nowhere near have their bodies pulled through the Earth. Every surface is decorated and covered with bones, and the old powers and magic of these bones is great. Undead walk its own halls, animated by nothing more then a desire to guard their own eternal resting place.
 There was once a council of dragons, the only time dragons got along at all, and they carved out a communal hoard. It was never used, and not a single copper piece was ever given freely by any of dragon kind. But the slaves of the dragons set up within the hoard, bringing along their own treasures as offerings, and their warriors and slaves await the day a dragon will come to master them.
 There is a tower on a lake, with one bridge. The bridge is guarded by three challenges; a watchmen who sees beyond the horizon, a sea serpent that can sweep anyone off the bridge, and flying stones from the tower's own battlements, that shatter all resistance. It's only once you get inside the tower that you realize it goes all the way down into a cave, the tower's original treasure stolen away and taken by the under world.
 The sewer under Sariia is a vast and efficient thing, with each new rain sweeps them clean. But the outflow goes down a tunnel into a dark, dark place. An entire lost world exists down there, the sewer their main food source, a feculent kingdom of crabs, rats, and flies. It is ruled by the maggot king, who gorges himself daily and believes that if he gets big enough before he molts, he'll become a God over all things that die and rot away.