Batman: Arkham Asylum Easter Egg - Hint to the Sequel

1. In the game, go to the Warden's Office located inside Arkham Mansion (preferably during the mission when you look for Dr. Young's notes, or after you finished the game).
2. When you walk through the door into the room with the fireplace, look to the left wall. On the center of the wall, spray the center of the wall with Explosive Gel three times.
3. Once you're done, just step away and detonate the gel. The explosions will blow open the wall and reveal an opening to a secret room (the room cannot be found on the map and the wall does not give you a hint that it's breakable). Inside the room, you'll find blueprints for an expansion of Arkham Asylum that will take up an entire city district. These are the blueprints for the setting for the upcoming sequel "Batman: Arkham City", which is coming in Fall 2011. Note that the map has been stamped "Approved".

Above the map are two pieces of concept art for the sequel.