It's hard to believe that one of the rarest and most expensive foods in the world is located by pigs and dogs. This exceptional fungus grows 3 to 12 inches underground near the roots of trees (usually oak but also chestnut, hazel and beech), never beyond the range of the branches. The difficult-to-find truffle is routed out by animals that have been specially trained for several years. Pigs have keener noses, but dogs are less inclined to gobble up the prize. Once the truffle is found, the farmer (trufficulteur) scrapes back the earth, being careful not to touch the truffle with his hands (which will cause the fungus to rot). If the truffle isn't ripe, it's carefully reburied for future harvesting. This methodically slow and labor-intensive harvesting method is what makes truffles so extremely expensive. Truffles have been prized by gourmets for centuries and were credited by the ancient Greeks and Romans with both therapeutic and aphrodisiac powers. A truffle has a rather unappealing appearance — round and irregularly shaped with a thick, rough, wrinkled skin that varies in color from almost black to off-white. Of the almost 70 known varieties, the most desirable is the black truffle, also known as black diamond, of France's Périgord and Quercy regions and the Umbria region of Italy. Its extremely pungent flesh is black (really very dark brown) with white striations. The next most popular is the white truffle (actually off-white or beige) of Italy's Piedmont region, with its earthy, garlicky aroma and flavor. Fresh imported truffles are available from late fall to midwinter in specialty markets. Choose firm, well-shaped truffles with no sign of blemishes. Truffles should be used as soon as possible after purchase but can be stored up to 3 days in the refrigerator. To take full advantage of their perfumy fragrance, bury them in a container of rice or whole eggs and cover tightly before refrigerating. The truffle fragrance will permeate the ingredients they're stored with, giving the cook a double-flavor bonus. Brush any surface dust off the truffle and peel the dark species (saving the peelings for soups). White truffles need not be peeled. Canned truffles, truffle paste in a tube and, to a limited extent, frozen truffles are also found in specialty stores. Dark truffles are generally used to flavor cooked foods such as omelets, polentas, risottos and sauces, like the famous Périgueux. The more mildly flavored white truffles are usually served raw by grating them over foods such as pasta or cheese dishes. They're also added at the last minute to cooked dishes. A special implement called a truffle slicer can be used to shave off paper-thin slivers and slices of truffle. Dishes flavored or garnished with truffles are often referred to as À la Périgourdine. See also chocolate truffle.