In Batman: Arkham City there are different ways to access or explore the outside of the game map. These areas in video games are commonly known as "Blue Hell".
Inside Arkham City Processing Center and in the Collapsed Streets there are at least three ways to explore the Blue Hell. You need the Remote Controlled Batarang to pass through non-solid walls of the map. In this way you can find unused parts of the game's map or see what's inside inaccessible rooms.
With this method you can see what's inside the famous locked room that many people believe is where Hugo Strange keeps his own Batsuit, as mentioned by The Riddler in an interview tape.
In the Processing Center you can also use the Remote Controlled Batarang on a non-solid wall to explore a hidden room. You can also see a glimpse of this room during the story, when Batman gain access to Wonder Tower to stop Protocol 10.
With the same method you can explore the Blue Hell in the Collapsed Streets. A similar non-solid wall can be found inside the Church \ Medical Center of Arkham City.
Inside the old GCPD building you can easily access the Blue Hell using a little trick. Use the Line-Launcher as shown in the video and explore the building from the outside.
Inside the Steel Mill, you can get in the Blue Hell by standing on a ledge and spamming the button to throw freeze grenades (RT on Xbox 360, R2 on PlayStation 3 and "5" on PC). In this way Batman will slowly gain altitude, so you can glide and get on the roof of Manager's Office. You can explore the Steel Mill from the outside and see part of Industrial District with low-quality textures.
Game available on: Sony PlayStation 3, Microsoft Xbox 360 & PC Video recorded on: PC
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