IN THE HENGDUAN mountains of Sichuan province, swollen brown rivers and trees heavy with ripe mangoes do not evoke digital wizardry. Yet until recently, there were buildings here with rack upon rack of specialised computers. They were often near hydropower plants that supplied them with electricity from dams. They needed lots of power. Their machines were used for “mining”, a process that involves validating transactions conducted in bitcoin and other digital currencies by solving cryptographic puzzles. In return, they received newly minted coins. The buildings were recognisable by their huge cooling systems: usually a wall on one side covered in...