Located in Collinsville, Illinois near the city of St. Louis, this largest pre-Columbian settlement north of Mexico is the pre-eminent example of a cultural, religious, and economic center of the Mississippian culture (800–1350), which extended throughout the Mississippi Valley and the south-eastern United States.

This agricultural society may have had a population of 10,000–20,000 at its peak between 1050 and 1150, which was equivalent to the population of many European cities at that time. It once covered more than 1,600 hectares and included some 120 mounds.

This makes Monks Mound roughly the same size at its base as the Great Pyramid of Giza (13.1 acres / 5.3 hectares). The perimeter of its base is larger than the Pyramid of the Sun at Teotihuacan. As a platform mound, the earthwork supported a wooden structure on the summit.

Unlike Egyptian pyramids which were built of stone, the platform mound was constructed almost entirely of layers of basket-transported soil and clay. Because of this construction and its flattened top, over the years, it has retained rainwater within the structure.

This has caused slumping, the avalanche-like sliding of large sections of the sides at the highest part of the mound. Its designed dimensions would have been significantly smaller than its present extent, but recent excavations have revealed that slumping was a problem even while the mound was being made.

According to whc.unesco.org. Source of photo: internet