Truffles are, in truth, mushrooms which grow underground in symbiosis with the roots of some trees. Only some animals are able to find them. Given its penetrating aroma, nowadays trained dogs are used to find them. The difficulty in finding them, along with a unique flavour, explain their high price on the specialised markets that sell them in Catalonia such as those in Vic or Centelles. Truffles are primarily found in April and June. They usually grow on rocky ground, exposed to strong winds and with trees such as oaks and holm oaks which can act as hosts. Referred to as 'the diamond of cuisine ' due to their exquisiteness, they are used as a condiment and also to flavour preserves and cured meat and sausages, such as the Catalan truffled butifarra. In modern Catalan cuisine truffles are also present in main courses, such as a bass with truffle, patatas enmascaradas with truffle or vegetables with truffle. The city of Vic has the only truffle market in the whole of Spain where this product is bought and sold. The price can go as high as 400 euros per kilo. The village of Olvan, in El Berguedà, holds the Truffle Fair in mid-December. This fair organises two contests for the seeker who finds the heaviest truffle and for trained dogs. Moreover, it is possible to take part in a tasting of the dishes made using truffles.