Derived from Pecorino Sardo, casu marzu goes beyond typical fermentation to a stage most would consider to be decomposition, brought about by the digestive action of the larvae of the cheese fly, Piophila casei. These larvae are deliberately introduced to the cheese, promoting an advanced level of fermentation and breaking down the cheese's fats. The texture of the cheese becomes very soft, with some liquid (called "lagrima") seeping out. The larvae themselves appear as transparent, white worms, about 8 mm (1/3 inch) long. When disturbed, the larvae can jump for distances up to 15 cm (6 inches), prompting recommendations of eye protection for those eating the cheese. Some people clear the larvae from the cheese before consuming; others do not.
However, Casu Marzu is quite real. It's been described in a number of newspapers and magazines including The Wall Street Journal and Bon Appetit. Taras Grescoe recently wrote about it in The Devil's Picnic: A Tour of Everything the Governments of the World Don't Want You to Try.
Apparently Casu Marzu isn't even the most disgusting food Sardinians eat. According to a 2004 article in Australian Magazine, that honor goes to 'tordi':
These are small, 10cm-long songbirds that feed on the island's plentiful myrtle berries. They are netted and poached, then served cold, three or four at a time, garnished with myrtle leaves. Their eyes are black, haunting, their necks spindly. They look like a plateful of baby dinosaurs. You are supposed to eat them whole - everything but the beak - in a few crunches.
If one is going to try some Casu Marzu, I think the perfect drink to wash it down would be some Army Worm Wine.