In the 15th and 16th century the high quality of the stone quarried at Bourré meant that it was used in the construction of great châteaux like Cheverny, Chinon and Chambord. By the 19th century there was a network of hundreds of kilometres of galleries, on multiple levels. The owner of one of the principal quarries started growing button mushrooms in worked out galleries as a sideline, and his family still own and operate the Cave des Roches. No more quarrying goes on and very little button mushroom cultivation. Once there were hundreds of caves in France producing mushrooms - now there are just 30.
In the 1990s new accelerated growing methods for button mushrooms were developed and many tonnes began coming out of Dutch greenhouses. The Delalande family at Bourré couldn't compete, so they decided to switch their production to more unusual gourmet varieties of mushrooms. Today they grow pied bleu, shitake, chestnut and grey and yellow oyster mushrooms which are sold to Michelin star restaurants all over the world. They also open the caves up to visitors.
Each of the varieties grown requires a slightly different approach. The costs of production vary from about €5 per kilo to €10 per kilo, but add the cost of transport, storage, packaging, retailer's margin and so on, and the price trebles or even more. The mushrooms become very expensive, which is why you don't see them in the supermarket. They are allowed to grow at a slow natural pace, resulting in a highly flavoursome, firm textured product (unlike the mass produced supermarket button mushrooms).
The pied bleu ('blue foot' or Wood Blewit Clitocybe nuda) is native to this area and can be found wild in the forest. However, grown in the caves they remain white and do not develop the violet colour, which is apparently a reaction to warm days. They also remain unmolested by fly larvae and slugs. Chefs love them because they retain a firm, almost crunchy texture when cooked, and will keep up to 10 days in the fridge in good condition.