Chichen Itza is an ancient city with almost 1000 years of history, one of the greatest Mayan centers of the Yucatán peninsula in Mexico. Many great buildings still stand between the Mayan ruins: the ancient pyramid El Castillo (30 meters/98 feet tall), the Great Ball Court, the Temple of the Warriors, the circular observatory El Caracol and more. Chichén Itzá was designated a UNESCO World Heritage site and also proclaimed to be one of the "New 7 Wonders of the World".
The ruins at Chichén Itzá are the most impressive and famous Maya ruins, but I personally prefer those at Tikal, Copan, and Palenque, which have a much more eerie atmosphere thanks to the surrounding jungle.