The Diablo 3 Secret Level - an evolution of the iconic Diablo 2 Secret Cow Level - has been found.
It's called Whimsyshire and it makes fun of the now meme-ified "Diablo 3 is too colourful" complaint.
Whimsyshire is a bright green cartoon world of purple ponies, presents, cupcakes, fluffy clouds and rainbows.
You can access Whimsyshire during Act One by gathering five ingredients and the Staff of Herding plan. But be warned, the cutesy creatures are of Act Four difficulty, and low level characters will apparently be demolished on even Normal difficulty.
Whimsyshire spawns random and elite bosses, and offers gold and item drops.
There's a video of the content below, and an explanation of how to get to Whimsyshire on Diablowiki.net.