Underground Observatory

18.8036°N, 99.2944°W.

The Underground Observatory at Xochicalco in the state of Morelos, Mexico, is a man-made cave cut into the natural bedrock beneath the acropolis of the Epiclassic city. The chamber was excavated between approximately 700 and 900 CE and features a vertical hexagonal shaft cut through the rock from the surface to the cave floor. Twice a year, on the days the sun passes through its zenith at this latitude — May 14 or 15 and July 28 or 29 — sunlight projects through the shaft and forms a perfect inverted image of the sun on the cave floor below. Outside these dates, a slit of light traces the sun's path across the floor as the year advances. The observatory was excavated by INAH archaeologists in the 1960s and 1970s and remains operationally accurate today.

The Underground Observatory is a building whose builders cut a hole through solid rock to project the sun onto the floor on two specific days a year.