El Caracol, Chichen Itza, Astronomical observatory in Chichen Itza, Mexico.

El Caracol is a cylindrical stone tower on a stepped platform, its lower half of pale rectangular blocks with a single dark doorway. Above a rounded cornice, the upper half is rough rubble stone, partly broken away, with faint carved panels and a domed top. A spiral staircase inside gives the structure its name.

El Caracol is dated to around AD 906, at the start of the Post Classic period of Mesoamerican chronology, by a stele on the Upper Platform. The Spanish name refers to the spiral staircase inside the tower that leads to its top, an unusual feature in Maya architecture. Its alignment with Venus, called Noj Eek' ("great star") by the Maya, reflects the astronomy practised at Chichén Itzá in the Post Classic period of Yucatán.

The alignment of the building to Venus and other celestial bodies shows how important astronomical observations were for religious calendars and agricultural cycles. Visitors can still see today the narrow window slits through which priests once tracked the movement of the stars.

Morning visits provide better lighting for photographs and fewer tour groups around the site. Access is through guided tours that start from Mérida or Cancún and include several ruins in Chichén Itzá.

A total of twenty different astronomical alignments have been identified in the architecture, marking specific celestial events and seasonal changes. In particular, the alignment with the extremes of Venus allowed precise predictions for religious festivals and harvest times.