Giant Bucket of Acid World is the place that Fairy World is unfortunately located directly above, which Jorgen von Strangle admits is not the best location but the rent is cheap. All though is was mentioned inPipe Down!, Timmy hasn't heard of it yet.
When Mr. Crocker stopped believing in Fairies, they began to lose their magical power, and Fairy World began to plunge into Giant Bucket of Acid World. They managed to restore power just before reaching the surface of Acid World (by having Timmy tell Mr. Crocker that he is the best teacher ever which then undoes the hypnosis Crocker was under, causing him to believe in fairies again, and restoring the power to Fairy World very quickly due to Crocker's spasming), and floated back up into the sky safely.
As its name suggests, the planet is shaped like a giant bucket filled with boiling and dangerous looking green acid.