Angkor, CambodiaNext to the road leading into Angkor Thom—one of the walled cities that make up Angkor, the ancient Khmer empire in Cambodia—stands a solitary but crumbling bridge with corbeled arches made out of recycled sandstone blocks. The bridge seems to be of little significance; no water runs beneath it. Few, if any, of the tour buses shuttling visitors to the temple grounds every day stop for tourists to have a look.

But to Dan Penny, an expert in environmental history who has studied the Angkor civilization for many years, the bridge tells an intriguing story. Its damage in the recent past, he says, is a reminder that while it was water—or the control of it—that built the city of