Some 1,600 years ago, the Temple of the Night Sun was a blood-red beacon visible for miles and adorned with giant masks of the Maya sun god as a shark, blood drinker, and jaguar.

Long since lost to the Guatemalan jungle, the temple is finally showing its faces to archaeologists, and revealing new clues about the rivalrous kingdoms of the Maya.

Unlike the relatively centralized Aztec and Inca empires, the Maya civilization—which spanned much of what are now Guatemala, Belize, and Mexico's Yucatán region (Maya map)—was a loose aggregation of city-states. (Read about the rise and fall of the Maya in National Geographic magazine.)...