Short answers, pulled from the story.
The Mississippi River Valley became the cradle for a transformation that began around 800 CE. Late Woodland societies abandoned tribal lifeways to adopt increasing complexity and sedentism. Production of surplus corn attracted rapid population concentrations in major centers.
Cahokia stood as the largest Pre-Columbian settlement north of Mexico with its massive Monks Mound rising over 100 feet high. These truncated earthwork platform mounds were usually square or rectangular structures built to support temples and burial buildings. Archaeologists found evidence of copper working at Mound 34 and astronomy alignments at Cahokia Woodhenge.
Maize-based agriculture supported larger populations and enabled craft specialization across the region. Shell-tempered pottery used riverine shells as tempering agents became a signature material for these communities. Widespread trade networks extended as far west as the Rocky Mountains and north to the Great Lakes.
Middle Mississippian cultures covered the central Mississippi River Valley and lower Ohio River Valley regions. South Appalachian Mississippian area sites distributed across Alabama, Georgia, northern Florida, and Tennessee adopted traits from northwestern neighbors around 1000 CE. Caddoan Mississippian territory included eastern Oklahoma, western Arkansas, northeastern Texas, and northwestern Louisiana.
Hernando de Soto led an expedition into the area during 1539, 1540 exploring villages throughout the Southeast. His chronicles became among the first documents written about Mississippian peoples and provided invaluable information on their practices. De Soto's later encounters left about half of the Spaniards dead along with hundreds of Native Americans.