France has probably the most expensive mushroom farms in the world. Built between 1930 and 1935, they cost then what is still a staggering pound;700 million. Lurking underground, they are damply sinister, massive structures - their 3.5m thick concrete walls far too strong for the cultivation of fungus.
But strong enough, the French government hoped back in 1930, to hold off an attack from Hitler's resurgent Germany. For what today houses mushrooms - and the occasional disco and museum - was then the key to the defence of France. It was still grieving for those lost in the trenches of the First World War. It did not want to fight. So it built the Maginot line.
Named after war minister Andre Maginot, the line with its linked concrete and steel bunkers was intended to protect France's land borders. Enough metal to build six Eiffel Towers was poured into the gun turrets and cupolas. Concrete that would have filled three-quarters of the Great Pyramid was used to construct the combat blocs. The result was 1,000km of tunnel and more than 100 forts, equipped with 500-1,200 men, guns, generators, hospitals, kitchens, bathrooms, and underground railways.
All the facilities needed to beat back Hitler's tanks. But there was a gap in the line, and in the government's strategy. It stretched only from Switzerland to Luxembourg. Then came the Ardennes Forest, considered impenetrable, and then came Belgium, a friend.
Unfortunately for France, one million Nazi troops and 1,500 tanks smashed through the impenetrable forest in May, 1940. The Allies were taken by surprise. They had moved their troops to protect Belgium, where a decoy German force had been sent.
By June 9 tanks and bombers had triumphed over defensive thinking. The Maginot line was cut off from the country it was supposed to protect. On June 15 the order came to evacuate.
The next time the forts were occupied in anger was by retreating Nazi troops. Today military enthusiasts sometimes amuse themselves planning direct assaults on the Maginot line. A harmless enough pastime, perhaps, though not as harmless as growing mushrooms.