A general view shows the 'Plaza de las Tres Culturas', or the plaza of the three cultures, in the central Tlatelolco area of Mexico City December 27, 2007. Archeologists have discovered the ruins of the 800-year-old Aztec pyramid in the heart of the Mexican capital that could show the ancient city is at least a century older than previously thought. The pyramid, found last month as part of an investigation begun in August, could have been built in 1100 or 1200, signaling the Aztecs began to develop their civilization in the mountains of central Mexico earlier than believed. (Henry Romero/Reuters)