Like a Russian nesting doll, the ancient Mayan pyramid of El Castillo is more than meets the eye.

Archaeologists studying the structure have long known that it contained a small pyramid within its imposing form. Using advanced imaging technology, however, they recently discovered an even smaller pyramid nestled within; one that may represent the oldest and best-preserved example of Mayan building from between 500 and 800 A.D.

"Given that no one has excavated this structure ... it is difficult to say with certitude if it is one of the oldest buildings at the site," University of California, San Diego anthropology professor Geoffrey Braswell told the Associated Press. “But this is quite possible.”

So far, archaeologists have discovered two pyramids hidden within El Castillo in the Mexican state of Yucatán. (Photo: Instituto Nacional de Antropología e Historia)

The researchers peeled back El Castillo's hidden layers using a technology called tri-dimensional electrical resistivity tomography or "ERT-3D." The electric pulses generated by the process revealed two hidden pyramids — with the innermost structure measuring 30 feet tall. It's estimated that each subsequently larger pyramid was constructed in stages roughly 300 years apart, likely in response to a shift in leadership.

"If we can research this structure in the future it could prove important, because it could tell us about the first-period inhabitants of the site," archaeologist Denisse Lorenia Argote told the AP.

The researchers believe the original pyramid may have been preserved due to its location over a sacred water source. That discovery, made in 2015, revealed a sinkhole measuring 82 by 114 feet and possibly up to 65 feet deep. It's hoped that the new map of El Castillo's interior may reveal a physical way to penetrate its inner temples and further reveal its secrets.