Swamp of Evil
The Swamp of Evil[1] is the Dark World equivalent to the Desert of Mystery. The swamp is inaccessible from the overworld map of the Dark World, and can be entered only by using the flute to summon the bird from Kakariko Village's weather vane. The Swamp of Evil is a dark and filthy place enduring a torrential downpour, and home to various swamp-dwelling creatures; here Link can find a cave with a Piece of Heart, and it's also home to the mysterious Ghost of Misery Mire. Ironically, the Light World equivalent of this rainy field is the Desert of Mystery, except the original entry paths do not exist in the Dark World. Instead, near where the exit to the overworld should be, Link can find a spot and use the Magic Mirror to warp to the Light World and find himself over a hill, where the Bombos Medallion can be gained.
The entrance to the Misery Mire can be found here. It can only be accessed by using the Ether Medallion on a platform with the medallion's sign on it.