Little has been known about what's inside "El Castillo," which has 365 steps — one for each day of the year — until now.
Scientists used the non-invasive tri-dimensional electric resistivity tomography technique, or "ERT-3D," to look inside. That's when they discovered a second substructure below the first one, according to CNN.
In other words, there's a pyramid within the pyramid!
The sub-pyramid is 10-meters-tall, and it is estimated that the original architects constructed it between 550 and 800 AD, according to Rene Chavez Segura, the lead scientist at the National Autonomous University of Mexico.
University of California, San Diego anthropology professor Geoffrey Braswell, who has conducted past research at Chichen Itza, told Fox News that the discovery may be new, but scientists may have detected the second pyramid in the 1940s.
"We know very little about this platform," Braswell said. "It appears to be much smaller than the outer two pyramids, and is not perfectly aligned within them."
One scientist compared this pyramid within a pyramid to a Russian nesting doll, with each layer encapsulating another.
National Institute of Anthropology and History research professor Denisse Argote told CNN that there may be more information — or other pyramids — inside this newest discovery.
"If this could be investigated in the future, this structure would be significant because it would speak to the first few periods of habitation of the site and would provide information about how the settlement developed," she said.
Now we're dying to see revisit Chichen Itza!