Archaeologists have discovered what may be the original structure built at the pyramid of Kukulkan at the Mayan ruins of Chichen Itza, experts said on Wednesday.

Last year, archaeologists using electrical imaging 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 on 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-metre (33ft) tall structure within the 20-metre (66ft) tall 'intermediate' pyramid that was covered over by the last construction stage, perhaps around 900 AD.

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]".

Ms 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 AD.