Cahokia, the urban center of the pre-Columbian Mississippian culture, was located near present-day Collinsville, Illinois. Several burial mounds and adobe structures were created in Southern Illinois across the Mississippi River from St. Louis. A gigantic mound, known as Monks Mound near Cahokia, is about the same height from its base as the Pyramid of Giza. Built around 1050 AD by an immense marshaling of human labor, this huge earth-work faced the site of a palisaded city that contained more than one hundred small artificial mounds marking burial sites. This Mississippi valley city of Cahokia is estimated to have had a population of about 16000 to 20000, the most concentrated population north of the Rio Grande until the late 1700s. Radiating out from Cahokia for many miles were tilled fields that supplied the corn for the urban dwellers. That civilization vanished circa 1400–1500 for unknown reasons. A severe earthquake that damaged Monk's Mound at that time might have challenged the supernatural powers claimed by the Cahokian chiefs. Cahokians might have also outstripped their water supply caused in part by large-scale deforestation, and the period of global cooling or mini Ice Age of the era might have caused recurring famines and migration.