Arkham Asylum's Underside, Arkham Wiki

Underneath Arkham Asylum consisted of caverns, an old sewer that emptied into the Gotham River, an unused Pump Station, and Killer Croc's Lair. The Caves also included a top-secret Batcave that Batman had built on Arkham Island. When the Joker began to take control of Arkham Island, the Arkham security guards were posted around the entrance to the Caves to the Pump Station, which had been closed by Warden Sharp years earlier on the grounds that it was a security threat. When Arkham East was over

Batman: Arkham Origins review - standing on the shoulders of giants, Games

Hall of Fame Review – Batman: Arkham Asylum (2009) – Last Token Gaming

Joker (Character) - Comic Vine

Bane - Batman Wiki - Neoseeker

Joker in other media - Wikipedia

Arkham Asylum, Batman: Arkham Wiki

Arkham Asylum, Arkham Wiki

Steam Community :: Guide :: You CAN play as the Joker on PC!

Serious Question: At what point in Arkham Knight did you realize it was Jason underneath the mask? : r/BatmanArkham

My Favorite Lesser Known Arkham Villains – ready for some slapstick?

Batman, Arkham Wiki

Alex Ross Batman at Batman: Arkham City Nexus - Mods and community

What year do Arkham games set in? - Quora

Killer Croc (Arkhamverse), Batman Wiki