This island, found on the southern most edge of any map, is an evil one. Sunshine doesn't fall on this jungle island, only small tidbits on the east side, where there is a beach, but on the west side, it's rock outcropping after rock outcropping, with dense jungle in the middle.
The dark Pokemon live on the west side of the island, transferring themselves to the spirit world like no one's business. And, to the east side of the island, psychic Pokemon reign. They sit there, meditating day and night.
Darkrai lives in a small meadow in the dead center of the forest, where no one dares to try and find. No light filters through in the forest, so it's almost impossible to find him. Mew rests in a huge tree on the east side of the island, too high for people to get to it. Mewtwo allows itself some solitary confinement, on the northern most point of the island, where a small cove can be found. He lives within that cove, to only be protected by the rock around him.