ZIGONG (自贡, zìgòng) a thriving industrial centre, has long been an important source of salt, tapped for thousands of years from artesian basins below the city. In the fourth century, the Sichuanese were sinking 300m-deep boreholes here using bamboo-fibre cables attached to massive stone bits. By the 1600s, bamboo buckets were drawing brine from wells bored almost a kilometre beneath Zigong, centuries before European technology (which borrowed Chinese techniques) could reach this deep. Natural gas, a by-product of drilling, was used from the second century to boil brine in evaporation tanks, and now also powers Zigong’s buses and taxis.