A while back I discovered a YouTube video explaining a glitch in Batman: Arkham City that lets you leave the Arkham City compound and fly around Gotham. Basically you settle onto a ledge, (I find that corners work best) and start throwing freeze bombs like crazy. Batman will rise higher with each throw, and after about 100 throws you can glide over the boundaries without being turned back. However, you can't land on anything in Gotham, so the higher you go initially, the farther you can glide, but eventually you'll drop below the ground into the abyss.
Below is good old Arkham Asylum from the first game. Obviously they didn't clear away all of Poison Ivy's plants, lazy bums.