Mayan Ruins can be found throughout the Yucatán peninsula and Central America. This large civilization flourished from 250 - 900 AD.
(Note: This is one of the civilizations that just disappeared for no reason. - RL)
(Note: Well they're still around, there are plenty of Mayans in that part of the world with their own culture. But yeah, the thriving civilization collapsed around a thousand years ago. People say it had to do with drought, but I don't think there's a general agreement what exactly happened. - DM)