"I will go no further," William Niven's Indian guide declared. "Beyond on every ridge, as far as you can see is all a part of one great City of the Dead . . . the gods will permit no man to go further and from here I return." Niven, however, did continue on and discovered a remarkable expanse of ruins in the rugged state of Guerrero along Mexico's western coast. During the early 1890s, Niven's explorations were sponsored by the American Museum of...