Archaeologists are unearthing a 2,000-year-old tunnel at Teotihuacan, the largest pyramid structures built in the pre-Columbian era, in the Basin of Mexico, searching for clues to the region's most influential former ancient civilization. Heavy rains at the site of Teotihuacan accidentally provided the first sign of the tunnel's existence in 2003, when the water made a tiny hole in the ground. – Photos by AFP