Monk's Mound is a massive pyramid-like earthwork in the ruins of the ancient city of Cahokia, in present-day Illinois. It was built by the Mississippians before 1,000 A.D., and was abandoned long before Europeans arrived in the area. Cahokia, including Monk's Mound, is now a UNESCO world heritage site.
According to the Cahokia Mounds State Historic Site official website, Monk's Mound (officially known as Mound 38 by archaeologists) is "the largest man-made earthen mound in North America." Archaeologists began making full-scale surveys of the site during the 1960s, when they showed that the base of the pyramid-like mound is over 1,000 feet across on its longest axis, and nearly 800 feet in the other direction. It consists of two terraces, climbing to a total height of about 100 feet. This is extremely unusual, since all the other Mississippian mounds, both at Cahokia and elsewhere, are smaller and do not have multiple terraces. This is a strong indication that the builders and owners of Monk's Mound occupied some sort of prestigious position in Cahokian society.
Unfortunately, Cahokia collapsed in the 1400s. The site was a ruin long before European missionaries were led to it by Native Americans in the 1700s. As a result, archaeologists and historians still know very little about the people who built and lived around Monk's Mound. According to UNESCO, it has been dated prior to 1,000 A.D. At its height, the city around Monk's Mound probably had a population of up to 20,000. Monk's Mound would have towered over the other 100 or so buildings and plazas. According to Sally Kitt Chappell of the University of Houston, it was probably either a temple or the ruler's palace, or perhaps both.
The history of the study of Monk's Mound at Cahokia dates to the mid-1800s. Amateur archaeologists had speculated about the nature of Cahokia for years, and Trappist monks even started a farm on the sprawling lower terrace. It was the Trappist farming project, and not any theory about what the mound was originally used for, which led to its present name of Monk's Mound. Ultimately, the site was privately purchased by Thomas Ramey in the 1860s so that it could be preserved and studied in greater detail. Even so, it was not until the 1960s that the largest series of excavations was organized by Nelson Reed. It was these excavations which turned up the true size of the city and confirmed the central, prestigious location of Monk's Mound.
Since then, researchers from the University of Milwaukee, the University of Illinois and Washington University have excavated parts of the terrace, turning up the original flooring, courtyards and patios. Today, archaeologists continue to study the site. Today's researchers, like Tim Schilling of Washington University, limit the amount of potentially destructive excavations by using advanced techniques like "stratigraphic observations and radiometric assays." These allow researchers to peer into the interior of Monk's Mound without risking serious damage to the structure.
The site as a whole, including Monk's Mound, was designated a state historic site in 1966 and a UNESCO World Heritage Site in 1982.