The most popular 'Easter Egg' in Grand Theft Auto 4.
Get in a cab and go to the Heli Tours (unlocked after first helicopter mission). Fly to Happiness Island and get close to the statue. There is a ledge you can only get to by helicopter. You must get so close your helicopter will hit the statue. Jump out above the ledge (you may get hurt).
Walk around until you are in front of the door with two plaques (one on each side). They read: No Hidden Content Beyond This Point. The door looks as if you cannot pass. Walk up to the door and you'll go through.
Climb up the ladder inside. When you reach the top, there will be a beating heart. It's pretty cool to look at, but that's it.