Flames spark in the Chicago barn of Patrick and Catherine O’Leary, igniting
a two-day blaze that kills between 200 and 300 people, destroys 17,450
buildings, leaves 100,000 homeless and causes an estimated $200 million
(in 1871 dollars; $3 billion in 2007 dollars) in damages.

Legend has it that a cow kicked over a lantern in the O’Leary barn and started
the fire, but other theories hold that humans or even a comet may have been responsible for the event that left four square miles of the Windy City and its
business district in ruins.