Lingqu Canal

This guide to the Lingqu Canal near Guilin opens around 214 BC, with half a million Qin soldiers stalled in the southern hills, worn down less by battle than by disease and by a supply line that couldn't cross the watershed between two rivers flowing in opposite directions. Emperor Qin Shi Huang's answer wasn't more troops, but an engineer named Shi Lu and a single order: make the water go the other way. You'll hear how he set about it, and how later dynasties kept adapting his channel for centuries after. Listen for the old saying locals still repeat about the wall in the north and the ditch in the south.