The illegal diggers come at night with shovels and sacks, hunting through the places where they know the professionals have been. They’re looking for the tonnes of ancient artefacts that lie hidden in Bulgaria’s soil. In the past two decades, Bulgarian law enforcement agencies say this plunder has turned into a €30m-a-year industry for local gangs, putting it a close third behind drugs and prostitution. The artefacts – gold Roman coins, ancient Greek silver, Thracian military helmets – wind up with falsified documents in auction houses in Europe and North America, or increasingly with wealthy Arab and Asian collectors.