Black Perigord truffles, such as the one above, vary in size from the size of a pea to the size of a tennis ball, and sell for in excess of $3,600 NZ per kg! They are quite temperamental to grow, requiring a stringently alkaline soil of pH 7.9-8.1. They like friable, well aerated soil, which is to be free draining, but moist a key times. The ground needs to be sterile of other competing fungi, with the absence of many certain tree species within a 50m radious., and they like their summers hot and their winters cold.
At harvest time the truffles can be found 10-30cm below ground, and are usually smelt out by trained hounds (although they may produce patches of brownish ground around each truffle if it is just below the surface (known as Brules).
Although they are traditionally cultivated in France and Italy (mainly central and southern France and Tuscany, where there is a high predominance of limestone soils), they have successfully taken off in parts of Australia, like Tasmania, and also in parts of New Zealand. Below is a small 3 part documentary on a truffle farm in NZ, shown on a program called Country Calendar.