Teotihuacan has been a mystery to Anthropologists. Very little is known about the original inhabitants and to this day no remains of these ancient people have ever been found. Teotihuacan long dominated central Mexico and had its apex between 100 B.C. and A.D. 750. It is believed to have been home to more than 100,000 people, but was abandoned long before the rise of the Aztecs in the 14th century. As the Aztecs rose in power, they eventually moved into the city and made the city theirs. This find could very well give us insight to a long forgotten people.
http://phys.org/news/2014-10-mexico-archaeologists-explore-teotihuacan-tunnel.html
