Monks Mound, Platform mound in Madison County, United States

Monks Mound is an earthen platform in Madison County, Illinois, forming the center of the Cahokia archaeological site. The structure extends over a base covering roughly 5.7 hectares and rises about 30 meters (100 feet) above the surrounding land.

Builders from the Mississippi culture began construction in the 9th century and expanded the structure in several phases into the 13th century. The Cahokia settlement lost its population between 1250 and 1350 and was abandoned, with the reasons for decline still being researched.

The mound's name comes from French Trappist monks who settled on a smaller nearby mound from 1809 to 1813 and grew wheat and garden produce on its terraces. Henry Brackenridge, who visited in 1813, published the first detailed description of the mound and named it after them. The Illiniwek community served by the River L'Abbe Mission on the south terrace was forced to leave around 1752.

Two straight flights of stairs are set into the grassy front face: a longer one rising from the lower left and a shorter one higher and further right, both leading to the flat summit. The surface is uneven grass with no structures, inscriptions or signs, and the mound rises from a flat lawn.

The base area surpasses that of the Great Pyramid of Giza, making this construction the largest pre-Columbian earthwork in North America by footprint. Archaeologists estimate that the construction required around 15 million baskets of soil carried by hand.