Truffles are a form of a mushroom it's self, but are quite different than mushrooms. They do grow under ground as regular mushrooms grow above. They take 3 to 4 years to grow which is a really long time for a mushroom. And this is why they are much more expensive. Buying an actual truffle can run you around $300 a pound. That's why I use truffle oil which is oil infused with truffle. You still get that awesome flavor of a truffle. If you would like to learn more Man Of Many has a great article on Truffles.