The great city, now archaeological site, of Teotihuacan was built 1,000 years before the Aztecs occupied it. It was one of the great cities of the world before 1400 and was inhabited by 100,000 people. It covered 8 square miles, but scientists claim that only about 5% has been excavated. No one is certain who built it, but it continues to be one of Mexico’s most popular attractions.
But now scientists are reporting that the main structure, the magnificent Pyramid of the Sun, maybe be in danger of collapse. Ironically, an attempt in the 1070s to preserve is might have backfired. Read more about it here