Cahokia was the largest Pre-Columbian city in what is now the United States, and peaked in size with a population of nearly 25,000 between the years 800 and 1500— bigger than the city of London at the time. Like the Great Serpent Mound, Cahokia was built by the Mississippian people and featured numerous earthen mounds—the result of a huge labor effort. There were around 120 mounds at Cahokia; the largest, known as Monk’s Mound, was one hundred feet tall, aligned to the sun, and possibly used as some kind of astronomical observatory in a manner similar to Stonehenge. Evidence of the city can be seen in Southern Illinois.