CHICHEN ITZA, MEXICO — A hidden third temple was discovered this month inside the Kukulkan Pyramid, one of the world’s most famous ancient structures.

Researchers used a 3D scanning technique called electrical resistivity tomography to find the 10-meter-tall structure inside the Temple of Kukulkan at Chichen Itza in the Mexico’s Yucatan Peninsula. Experts believe this third structure was constructed between the years 550 and 800, reported the BBC, citing researchers.

Also known as El Castillo, the Kukulkan was already known to have been made of up other two temples. According to an AFP report, these consist of a 30-meter-tall external structure with another 20-meter-tall pyramid inside of it. It is thought that the new structures were added to cover decaying ones or to welcome new leaders.