In the 1930's, scientists discovered that El Castillo, encased a second, 20-meter tall pyramid , which dates back to approximately 850-900 AD within it. When archaeologists tunneled into the pyramid to examine the inner structure, it revealed an underlying third pyramid. In 2016, using a scanning technique known as tri-dimensional electrical resistivity tomography, or ERT-3D, scientists were able to map that inner pyramid's dimensions, which they date to between 600 and 800 AD, and believe it have been about 10 meters high with a flat roof and rectangular base. In 2014, a partially water. El Pirámide de Kukulcan (Pyramid of Kukulkan), first identified by Spaniards as El Castillo (the Castle), was built between 900-1000 AD. Dominating the North Platform of Chichén Itzá, the step-pyramid stands about 24 meters high, on a rectangular platform about 55.5-meters wide, and consists of a series of nine square terraces, each approximately 2.57 meters high with a 6-meter high temple upon the summit. Each side of the pyramid has a large staircase, 91 steps per side, and one more leading to the upper temple, giving it 365 steps--one per day of the year. Chichén Itzá, a large Pre-Columbian city built by the Maya people of the Terminal Classical period in the Northern Maya Lowlands, is one of the most visited archaeological sites of Pre-Hispanic Mesoamerica. A religious, military, political, and commercial centre, at its peak, it is thought to have home to 35,000 people. The site has been designated a UNESCO World Heritage Site and was voted as one of the New 7 Wonders of the World. The city, centered around two cenotes (chi meaning "mouths" and chen meaning "wells"), was initially during the 6th century. An influx of foreign influence during the 10th century--perhaps through a conquering people, or a migration--particularly by the Toltec of Central Mexico, led to the construction of the major buildings on the site--El Castillo, the 24-mneter tall pyramid Temple of Kukulcán; the ball court, and Temple of Warriors and Group of a Thousand Columns--probably during the Early Post Classic Period (c. 900-1200). In the Late Classic Period (1200-1540), Chichén appears to have been eclipsed by the rise of the city of Mayapán and the city was left to the jungle when the Spanish conquered in the 16th century. Excavation began in the 19th century, and the site became one of Mexico’s prime archaeological zones.