Chichen Itza, Mexico

The El Castillo pyramid at Chichen Itza, built by the Maya around 800-900 AD, demonstrates extraordinary astronomical precision. Its four stairways of 91 steps each, plus the top platform, equal 365 — one for each day of the year. At the spring and autumn equinoxes, a serpent shadow appears to slither down the northern staircase. The entire complex is an elaborate stone calendar.