The Maya Observatory
El Caracol is the Spanish name for "The Snail" and the name was given to this structure because of its spiral inner staircase similar to the detail of the inside of a snail's shell.
Though its current eroded state gives the main building the appearance of a modern day observatory, (with the large domes roof) the actual shape of the non-eroded building was a square-cylindrical top and it has been eroded to this shape over many years.
The Maya were accomplished astronomers and it is believed El Caracol was used to observe the moon, stars and the planet Venus. The main structure of the observatory sits atop a raised platform.
-- Chris Reeves