The Great Chicago Fire

The first Europeans to discover the land that is now Chicago were Loui Jolliet and Jesuit Jacques Marquette in 1673. The first permanent settlement was founded by jean Baptiste Point du Sable, an African American from Santo Domingo, in 1781. The settlement was attacked regularly by Native Americans until 1832, when Chief Black Hawk was defeated at Fort Dearborn. Chicago was granted a city charter by Illinois and incorporated on March 4, 1837.

The Home Insurance Building, the world's first skyscraper.

A great fire ravaged the city in 1871. Most of the city was burned to the ground. Though this may seem bad, the authorities used this fire to correct several mistakes the city had. This eventually led to the world's first scycraper in 1885.

By the turn of the century, the city's drinking water had become contaminated. To solve the problem, city engineers reversed the flow of the Chicago River that dumped sewage into Lake Michigan. This led the Chicago River into the Illinois River and allowed the lake to become clean and safe for drinking water.

Information from Wikipedia.