Truffles are subterranean fungi that grow in a symbiotic relationship with the roots of specific trees, such as oak and hazelnut. The 'Périgord Black Truffle' is one of the most expensive food items in the world. Truffles are difficult to find because they grow entirely underground. Historically, female pigs were used to find them because truffles produce a compound (androstenol) that is very similar to a pheromone found in male pigs. Today, specially trained dogs are more common because they are less likely to eat the truffle once they find it. The intense, earthy aroma of truffles comes from volatile organic compounds like dimethyl sulfide. Because they cannot be easily farmed and must be foraged, truffles can sell for thousands of dollars per pound.