A vast city built by the ancient Maya and discovered nearly a century ago is finally starting to yield its secrets. Excavating for the first time in the sprawling complex of Xultún in Guatemala’s Petén region, archaeologists have uncovered a structure that contains what appears to be a work space for the town’s scribe, its walls adorned with unique paintings — one depicting a lineup of men in black uniforms — and hundreds of scrawled numbers. Many are calculations relating to the Maya calendar.