In which midwestern U.S. state is this historical site located, an ancient Columbian Native American city?

The Cahokia Mounds State Historic Site is located in the midwestern state of Illinois directly across the Mississippi River from modern St. Louis, Missouri in the United States. It is the original site of a pre-Colombian Native American city that existed around 1050 to 1350 CE.

Covering an area of 2,200 acres (890 ha), or about 3.5 square miles (9 km2), the site contains about 80 mounds, The ancient city located here however covered a much larger area. At its apex around 1100 CE, the city covered about 6 square miles (16 km2) and included about 120 man-made earthen mounds in a wide range of sizes, shapes, and functions.

It is estimated that the population at the apex was 14,000 - 18,000 people. Cahokia was the largest and most influential urban settlement of the Mississippi culture, which developed advanced societies across the central and southeastern United States, beginning more than 1,000 years before any European contact.

It is one of the 24 United Nations Educational, Scientific and Cultural Organization World Heritage Sites within the U.S.

The inhabitants left no written records beyond symbols on pottery, shell, copper, wood, and stone, but the elaborately planned community, wood hedge, mounds, and burials, reveal a complex and sophisticated society.

The city’s original name is unknown. The mounds were later named after the Cahokia tribe, a historic Illiwek people living in the area when the first French explorers arrived in the 17th century.