While the Great Chicago Fire gets all the fame and notoriety, a far worse fire (actually firestorm) occurred the same day. October 8, 1871.
While the Chicago Fire claimed 300 or so lives and burned about 3.3 square miles, the Peshtigo Fire claimed somewhere between 1500 and 2500 victims and burned an area roughly the size of two Rhode Islands (roughly 2.3 million acres of fire damage). To this day it is the worse fire in American history.
There were actually several fires on that day. The summer had been hot and dry with a drought. That day brought strong winds and most likely the Peshtigo fire was started by an unattended logging camp or an attempt at a controlled burn. Wisconsin and Michigan were timber country in the late 1800s and going through a boom, so everything was wood and it burned.
During World War II the US government studied the Peshitgo fire to learn how to create firestorms that later destroyed Dresden and parts of Tokyo.