Between 1840 and 1940, over two million people departed from a handful of stone jetties along the Tan River. They were leaving Wuyi (五邑, wǔyì — the five counties of Taishan, Kaiping, Xinhui, Enping, and Heshan), a stretch of rocky, typhoon-prone coastline on Guangdong’s southwestern edge, bound for gold fields in California, tin mines in…