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

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