Look under the map in GTA 4: The Ballad of Gay Tony

Now this glitch is out there! It is contained in Grand Theft Auto 4: The Ballad of Gay Tony, and enables you to clip yourself under the map enough to at least look at the underside, if not walk there. All you need is a helicopter and some patience! Your helicopter will be destroyed, so make sure you're at the point in the game where you can get another easily before you start messing with this!