The Grand Canal

The Sui dynasty reunified China and then linked its rice-growing south to its hungry, militarized north with over a thousand miles of canal, conscripting millions to dig it. Enormous numbers died, the labour and the wars that followed broke the dynasty within a generation, and the Tang inherited both. But the canal held China together economically for the next thirteen centuries — a single dynasty destroyed itself building the thing that made every later dynasty possible. Parts of it still carry freight today.