The flat river bottomland of the American Bottom — the floodplain of the Mississippi River near its confluence with the Missouri and Illinois rivers — should be flat.

It is flat, mostly.

But rising out of that flatness are shapes that do not belong to any natural geological process: great squared-off platforms of earth, stepped terraces, conical peaks, ridges that run in straight lines for hundreds of yards.

More than 100 earthen mounds survive within the boundaries of the protected site.

Before European contact and the agricultural and industrial development that followed, there were somewhere between 120 and 200 mounds across the Cahokia complex.

The largest of them is Monks Mound, named not for any Native tradition but for a group of Trappist monks who settled near it briefly in the early nineteenth century and grew gardens on its lower terraces.

Monks Mound is the largest pre-Columbian earthen structure north of Mexico.

It rises approximately 30 meters — about 100 feet — above the surrounding plain.

Its base covers an area of about 5 hectares, or roughly 12 acres.

It contains an estimated 814,000 cubic meters of earth, all of it moved and placed by human hands, without metal tools, without the wheel, without draft animals.

The construction of Monks Mound was not a single project.

Archaeological analysis has shown that it was built in at least fourteen construction stages over a period of more than three centuries, with the central platform gradually raised and the terraces added and modified over time.

The mound’s builders moved earth in baskets, carried by workers who made the trip from the borrow pits — the areas where earth was excavated — to the mound site and back again, thousands and thousands of times.

The effort required, calculated by archaeologists using estimates of the volume of earth moved and the carrying capacity of basket-based transport, was staggering.