Final answer:
At the turn of the 19th century, there were 120 mounds in present-day St. Louis, part of the pre-Columbian Cahokia complex across the Mississippi River, which was once a bustling metropolitan center.
Step-by-step explanation:
At the turn of the 19th century, near present-day St. Louis, the most significant pre-Columbian settlement was Cahokia, located just across the Mississippi River. This archaeological site, dating from around 600-1400 CE, consisted of a complex with 120 mounds. The Cahokia complex included vast community plazas, a woodhenge for astrological observations, and tremendous earthen constructions such as Monk's Mound.
Monk's Mound, notable for being the largest of the mounds, is about ten stories high and covers 13.8 acres at the base. The top of this imposing structure likely served as the heart of the city, possibly hosting either a grand temple or the residence of the city's paramount chief. The mounds were central to the social and religious life in Cahokia, marking it as a hub of political and trading activities along the Mississippi River.
The construction of Cahokia's mounds was an enormous endeavor, involving the movement of approximately 55 million cubic feet of earth. This emphasizes the sophisticated nature of their society and the organizational skills necessary to marshal such a workforce. Despite its splendor, the civilization at Cahokia began to decline after 1300 CE, due to reasons which may include environmental or social pressures.