Archaeologists using electrical imagining techniques found that the pyramid Temple of Kukulcan at Chichen Itza, which is also known as El Castillo, was built atop two other structure that were possibly earlier pyramids.
The newest discovery suggests that the pyramid was built in three phases. The recently-discovered smallest pyramid was constructed between the years 550-800 AD. The middle structure was built in the years 800-1000 AD, while the largest one is believed to have been finished between 1050-1300 AD.
Both inner structures were detected using what is called three-dimensional electric resistivity tomography, or ERT-3D. But it is possible that the structure of the third, earliest pyramid was found in 1940s when archaeologist conducting excavations found a third platform under the main pyramid. According to the researchers the early structure appears to have a staircase and perhaps an altar at the top that may have just been filled in and preserved. At this point it is not clear whether it will be excavated.
(after PhysOrg, Associated Press, BBC News & Universidad Nacional Autónoma de México)