Built in about A.D. 160, Rockwell Mound is thought to be the largest prehistoric earthwork in the Illinois River Valley and the largest known mound built by Indians of the western Hopewell, or Havana, culture. Found along major rivers of the Midwest, mounds of the Havana culture were usually built over the log-covered tombs of prominent leaders, and ceremonial and everyday items were often placed with the burials. This two-acre, 14-foot-high mound has been estimated to have required about 1,700,000 basket loads of earth to construct. Because of its size and strategic location opposite Spoon River, it was probably the most important of the Havana site's more than twenty mounds. Havana was a trading and ceremonial center with trade routes that spanned much of the midcontinent. The mound was not scientifically verified until 1986, when a small test trench yielded pottery fragments and a variety of other identifiable material, and individual basket loads of earth were clearly visible in the trench walls. The site was added to the National Register of Historic Places in 1987.