Image copyright AP Image caption Researchers used 3D imaging to illustrate their find 
 
 A third structure has been found within the famous Kukulkan pyramid in eastern Mexico, experts say. 
 
 The 10m (33ft) tall pyramid was found within two other structures that comprise the 30m pyramid at the Mayan archaeological complex known as Chichen Itza in Yucatan state. 
 
 The discovery suggests that the pyramid was built in three phases. 
 
 The Mayan civilisation occupied Central America and had its peak around the 6th Century AD. 
 
 The recently-discovered smallest pyramid was constructed between the years 550-800, researchers say. 
 
 The middle structure was discovered in the 1930s and is estimated to date back to the years 800-1000, while the largest one is believed to have been finished between 1050-1300. 
 
 "It's like a Russian nesting doll. Under the large one we get another and another," researcher Rene Chavez Seguro told a news conference. 
 
 Image copyright AFP Image caption The Kukulkan pyramid dominates a major Mayan archaeological site 
 
 Image copyright AFP Image caption The 10m (33ft) tall pyramid was found within two other structures that comprise the pyramid 
 
 Pyramids were built within each other for various reasons, including deterioration of the structure or the arrival of new leadership, experts say. 
 
 They say the smallest pyramid was discovered using 3D Electrical Tomography, a non-invasive technique that measures the electrical signal caused by small injected currents to form a representation of the pyramid's interior without causing damage. 
 
 The discovery could provide new insights into the original Mayan culture before it intermingled with populations from central Mexico. 
 
 Last year, archaeologists discovered that the Kukulkan pyramid was built on top of a cenote, or underground river, which are common in the region and revered by the Maya. 
 
 Today Chichen Itza is a Unesco World Heritage Site, and receives more than a million visitors a year. 

 MEXICO CITY -- Archaeologists have discovered what may be the original structure built at the pyramid of Kukulkan at the Mayan ruins of Chichen Itza, experts said Wednesday. 
 
 Last year, archaeologists using electrical imagining techniques found that the pyramid, which is also known as El Castillo, was built atop a subterranean river, or a cenote. 
 
 Archaeologists have long known that a smaller pyramid is encapsulated underneath the visible temple. 
 
 Researchers said Wednesday that they had detected an even smaller structure inside the other two structures. Using what is called tri-dimensional electric resistivity tomography, or “ERT-3D,” they found a 10-meter (yard) tall structure within the 20-meter tall ‘intermediate’ pyramid that was covered over by the last construction stage, perhaps around 900 A.D. 
 
 Archaeologist Denisse Lorenia Argote said “if we can research this structure in the future it could be important, because it could tell us about the first-period inhabitants” of the site. 
 
 Argote, of Mexico’s National Institute of Anthropology and History, said the first structure may be in the “pure Maya” style from between 500 and 800 A.D. 
 
 University of California, San Diego anthropology professor Geoffrey Braswell, who was not involved in the latest project but who has conducted research at Chichen Itza, said the discovery may be new, or may be a structure detected in the 1940s. 
 
 Braswell said that while digging into the intermediate-layer pyramid in the 1940s, one archaeologist found a third platform buried within it. 
 
 “The tunnel was unstable, so we know very little about this platform,” Braswell wrote. “It appears to be much smaller than the outer two pyramids, and is not perfectly aligned within them.” 
 
 The computer image distributed by the researchers also showed underlying structure not quite aligned with the subsequent layers. 
 
 Braswell compared the Kukulkan pyramid to a Russian nesting doll, with each layer encapsulating another. But at the bottom, there may be more than one platform encapsulated. 
 
 “To make matters more complicated, “ Braswell wrote, “the third Russian doll moving in may actually be one of a set of several small dolls rattling around inside the same shell. We just do not know. “ 
 
 Rene Chavez, a researcher at the National Autonomous University’s Institute of Geophysics, said the early structure appeared to have a staircase and perhaps an altar at the top that may have just been filled in and preserved. The structure has been mapped, but it is not clear whether it will be excavated. 
 
 “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,” Braswell said. “But this is quite possible.”
– On the surface, the Kukulkan temple that sits among the ruins of Chichen Itza looks nothing like a Russian nesting doll, but it's essentially the pyramid equivalent, scientists say. They'd known for decades that the 100-foot-tall structure in Mexico's Yucatan state sits on top of a smaller, 65-foot-tall pyramid. Now, a new scan reveals that a third pyramid is beneath the second one, reports the BBC. Hence the Russian nesting doll comparison: "Under the large one we get another and another," says project head Rene Chavez Seguro. The outer pyramid was built between 700 and 1,000 years ago and draws 1 million visitors a year. This newly discovered pyramid, reaching 33 feet tall, may be up to 1,500 years old and appears to have a staircase and an altar at its peak. It's also slightly out of alignment with the other two pyramids, which "may indicate that there are other buildings hidden at the same level," reports Deutsche Welle. As for why the Mayans built over it, the BBC cites two possibilities: a transfer of leadership or because it was simply in bad shape. CBS News reports the discovery was made using a non-invasive electrical imaging technique called 3D Electrical Tomography, and says no excavation plans have been announced. (This Mexican "mountain" is actually a stack of six pyramids or more.)