Once you gain access to Happiness Island, find the Helicopter Tours, which is directly east of Happiness Island, and steal a helicopter. Fly to Happiness Island and over the Statue Of Happiness, then jump out of the helicopter at the statue's feet. You should land on the top-most tier of the statue, which is basically a square platform with a door in the center of each side. Run around the platform until you see a door with a plaque on either side of it that reads, "No Hidden Content Here". It may seem like you cannot go through the door, but you can -- it doesn't open, you simply walk through the door. You will find an empty room with a tall ladder inside. Climb it, and when you reach the top, look up to see a huge beating heart, held in place by chains.