The Secret Place is a cave hidden behind the island's waterfall, where the children visited during their childhood. Sora and Riku first discovered it when they were young children. There is a door in the cave which is actually the Keyhole of the world. The cave's wall are covered in chalk drawings, some hinting at the earlier encounters of the children . Among them is a drawing of Sora and Kairi sharing a Paopu Fruit, as well as drawings of Donald, a Chocobo, Disney Castle, and a monster coming out of a swirling ball. There's also a drawing on which you can see a dog facing a kid , this is most likely referring to the point where Pluto finds Sora in Traverse Town. Riku enters here near the beginning of the game and opens the door, unleashing the Heartless onto the islands. Whether he did so knowingly and willingly or not is not known, though it is likely that the words of Xehanort influenced him to do so.