If you stand at the foot of one of the contemporary skyscrapers on Avenida Reforma or in the neighborhood of Santa Fe, it's easy to forget that people have been building in this city for centuries. For those who need a reminder, the Templo Mayor, an active archaeological site just off the Zócalo in the Centro Histórico, is an obligatory visit. Templo Mayor, a UNESCO World Heritage site, was built in the 1300s and one of the main temples of the Aztecs' capital city, Tenochtitlán (now modern-day Mexico City). While occasional discoveries were made on the site in the early and mid 20th-century, it wasn't until the late 1970s and early 1980s that excavation began in earnest after a team of electricians accidentally discovered a massive disk—nearly 11 feet across and weighing more than eight tons—that turned out to be an Aztec artifact. Since then, thousands of objects have been discovered; many of them are housed in an on-site museum. The temple and other structures continue to be excavated and studied by researchers today.