On October 8, 1871, flames sparked 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;
roughly $4 billion in 2021 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, including its business district, in ruins.

Dry weather and an abundance of wooden buildings, streets and
sidewalks made Chicago vulnerable to fire. The city averaged two
fires per day in 1870; there were 20 fires throughout Chicago the
week before the Great Fire of 1871.