Teotihuacan, Mexico [Photo/Xinhua]

Teotihuacan is a pre-Columbian city located within the Basin of Mexico, 30 miles northeast of modern day Mexico city. Known as an important site for the Indian civilization, the city is thought to have been established around 100 BC and continued to be built until about 250. It may have been an "active" city until some point between the 7th and 8th centuries.

When the city prospered around the 6th and 7th centuries, it had a population of about 200,000 -- the equivalent to the population of Chang'an, a major Chinese city in ancient times. It was included as one of the UNESCO World Heritage Sites in 1987. However, who exactly lived in this mysterious ancient city is still unknown.