Instructions
Travel to "Quest Island," the last of the seven Sevii Islands. This island can be accessed by the ferry in Vermilion City.
Enter the cave in Sevault Canyon. Sevault Canyon is the furthest south stretch of land on Quest Island.
Move the boulders into the holes with the Pokemon move "Strength."
Pick up the item on the ground called the "Tanoby Key."
Walk out of the cave and head to the southernmost point of the island.
Use the move "Surf" to move on the water. Swim to the caves located east and west of the island.
Enter the cave and catch Unowns. Each of the seven caves have their own name and only contain specific Unowns.