The Melon Cave is the underground lair of Captain Melonhead and his sidekick Splinter the Wonderwood. It was first mentioned in "Robbin' Ed" but made its debut in Ed, Edd n Eddy's Big Picture Show and is a parody of The Bat Cave.
Jonny accesses the cave by climbing up a tree branch, and pressing a secret button hidden inside a fake bird's egg. The button causes a door inside the tree's bark to open which allows access to the cave. The "cave" is far larger than it appears. It has a large melon-logo printed on the floor, it has a melon-peel styled staircase that circles the edges of the room with each step lighting up when individually stepped on, and it also contains a machine that transforms Jonny and Plank into Captain Melonhead and Splinter the Wonderwood.
The room is lined with lamps (complete with melon-like bases, lamp shades, etc.) to illuminate it.