Cahokia Mounds is located just east of St. Louis, MO. This was the last of the USA world heritage sites that I visited. The natural site was very interesting. Some of the mounds are more than 100 ft. tall. The site contains at least 70 mounds that are between 800 and 1000 years old. The entire site (a large residential city of the Mississippean Native Americans) was almost completely surrounded by a very impressive fortress.
The museum at the mound site is quite good. It has actual artifacts unearthed at the site as well as representations of the city. Several grave sites have been excavated here. One, a high ranking man, was wrapped in a cloak covered with more than 10,000 round shells sewn into the shape of an eagle.
The site is all the more interesting since it presents a fascinating contrast between present day Mississippi river life and that of a millenium ago. (You can see downtown St. Louis and the Gateway Arch from the top of the Monk's Arch)