Spleen Island is an island located in the middle of a lake named after the fearsome pirate, Badbeard. It was the site of Badbeard's booty of treasure until Phineas, Ferb, Isabella, Buford, Baljeet, and the Fireside Girls found it. It also housed Dr. Doofenshmirtz's new evil lair (which apparently got destroyed after Candace hit the self-destruct button).
To get on Spleen Island, you must sail from the "Point Plots", through the "Sea of Bile", then through the "Stones of Gall", and then into the "Cove of Incontinence". After that, you can continue on foot into the "Dark Tunnels of Doom".
Spleen Island has many hazards and is filled with many monsters such as giant iguanas.