Many of the factories that once dotted the outskirts of French cities such as Paris, Lille and Marseille have long been closed. But scars remain: pollutants in the soil including petrol, solvents, cyanides, heavy metals and even traces of radioactivity. There are nearly 7,000 former industrial sites that are polluted or potentially polluted in France and another 300,000 sites still need to be assessed. These wastelands are often gold mines for developers and can also help reduce urban sprawl. But reclaiming this contaminated land is costly and rarely a simple exercise.