October 8, 1871. According to legend, the fire was sparked when a cow tipped over a lantern in the barn of Catherine and Patrick O'Leary. The fire burned for almost three days. The actual cause of the fire is unknown. Due to numerous wooden buildings, sidewalks and streets, four-square miles of Chicago was devastated by the blaze. As many as 300 people were killed. More than 17,000 buildings were destroyed. 100,000 people were made homeless. But reconstruction of the city allowed for modern planning, including the world's first skyscraper buildings. Less than 10 years after the fire, Chicago boasted a population of 1.5 million. The city had become a major transportation and economic hub.