Modern-day archaeologists are still unraveling the mysteries of Teotihuacan. For instance, large, human-made cave-like structures were discovered 20 feet (6 m) under the Pyramid of the Sun with an amazing treasure of artifacts.
In addition, a 338-foot-long (103 m) human-made tunnel containing a large quantity of liquid mercury, jade statues, bones of jaguar, carved shells, rubber balls and other artifacts were found under the Pyramid of the Feathered Serpent.
For reasons unknown, Teotihuacan and its amazing culture collapsed in the years after A.D. 600. Many of the great city’s structures were razed, burned and left for centuries in ruin. Over the decades, the natural elements of rain, wind and heat slowly eroded the ancient city.
In 1897, the Mexican government took control of the ruins of Teotihuacan. Even though Mexico’s National Institute of Anthropology and History began overseeing the preservation of Teotihuacan in 1939, the immediate area around Teotihuacan — including many acres (1 acre is 0.4 hectares) of yet-to-be excavated parts of the ancient city — are under threat today due to modern expansion, including a highly controversial Walmart store and booming firework displays.
Teotihuacan was designated a World Heritage site by the United Nations Educational, Scientific and Cultural Organization (UNESCO) in 1987.