Mayan Observatory

Chichen Itzá, Mexico.
El Caracol ("The Snail") is located to the north of Las Monjas. It is a round building on a large square platform. It gets its name from the stone spiral staircase inside. The structure, with its unusual placement on the platform and its round shape (the others are rectangular, in keeping with Maya practice), is theorized to have been a proto-observatory with doors and windows aligned to astronomical events, specifically around the path of Venus as it traverses the heavens.
It was explored between 1923 and 1931 by the Carnegie Institute which detected a series of six superimposed constructions. It is believed that its construction dates from an intermediary period between the Terminal Classic and the Early Post Classic periods. The large trenches on the upper platform are samples of consolidated excavations used for didactical purposes.