You've surely heard the story of the Great Chicago Fire (which wasn't really started by a cow). Strangely though, the fire really wasn't that big—at least when compared to many of the others that broke out on that same terrible night of October 8, 1871.
The story you probably haven't heard is how that 24-hour period was the most fire-ridden day in American history. The same day as the Great Chicago Fire, another blaze, the Peshtigo Fire, was burning throughout rural Wisconsin.
The Peshtigo Fire may not be as famous as the one in Chicago, but it still holds the record as the most deadly blaze in American history, killing somewhere between 1,200 and 2,500 people. Three other fires burned through the Midwest as well that night, after a particularly hot and dry day occurred during an already bad heatwave and drought. Read the whole story, including a crackpot theory involving a lost comet, over at io9.