During World War I, 1.5 billion bombshells were fired on a single frontline extending 700 kilometres from the North Sea to the French border with Switzerland. A third of them never exploded and many of them remain buried. A century later, France is still in an unfinished battle to erase these enduring scars. It could take more than 300 years to rid the soil of this toxic waste, as forgotten shells continue to pose a silent threat to humans and the environment.