One hundred and fifty years ago today, on October 8, 1871, the Great Chicago Fire ignited. Burning over a period of two days, the blaze destroyed thousands of buildings over an area of three square miles; killed an estimated 300 people and left over 100,000 more homeless: and caused an estimated $200 million in damages, not to include the looting and general lawlessness that occurred in the immediate aftermath. Even now, all these years later, the exact cause of the inferno is unknown. But it hasn’t stopped a rotating number of ideas and culprits from taking the blame:  

The most famous theory, of course, is the kicking of the lantern by Mrs. O’Leary’s cow. The story says that Mrs. O’Leary was in the barn behind her southwest side cottage at 137 DeKoven Street milking her cow at 9 PM when something startled the animal; it responded by kicking over the lantern and sparking an inferno that spread rapidly. Although the tale was widely accepted, Mrs. O’Leary herself steadfastly denied it, claiming she never milked after dark and was instead merely a convenient scapegoat. It’s interesting to note that, although many maintain the fire started here, it did not destroy O’Leary’s cottage, mere yards from the accepted flashpoint.

Another related theory involved the same barn but a different culprit: the O’Leary’s neighbor, Daniel Sullivan, known around town as “Peg Leg” due to his missing limb. Rumor had it that Sullivan snuck into the barn to enjoy a smoke and a nightcap when a stray spark from his pipe ignited a mound of hay. The flames spread so quickly, he barely escaped with his life.

There is still a third theory connected to the O’Leary property, one that involves underage gamblers and a bit of boyhood mischief gone south. The will of one Louis Cohn, who died in 1942, contained a confession he’d been too ashamed to share beforehand: he maintained that on the night of the fire, he and Mrs. O’Leary’s son, along with a few other boys, had been shooting craps in the hayloft by lantern light that night, when one of the boys—not the cow—knocked the lantern over. As the fire spread, the boys fled–though not without first grabbing their money.

There was enough evidence to support these alternate explanations that Mrs. O’Leary and her cow were posthumously exonerated by Chicago’s Committee on Police and Fire in 1997, though neither Sullivan nor Cohn have been officially blamed in their stead.

There were, however, other conjectures about the cause of the blaze. Citing the bluish flames reported by several fireman, some believed that pieces of Biela’s Comet—a periodic comet that apparently disintegrated around the time of the fire—might have fallen to Earth and started the fire. They cited another destructive fire, started that same day, as evidence: the wildfire in Peshtigo, Wisconsin, which ultimately ended up claiming over 1,000 lives. Most discount this as a tragic coincidence, though, as the Peshtigo fire is believed to have been caused by careless railroad workers.

Perhaps the most unsettling rumor, though, was that the fire was intentionally set. According to the Chicago Tribune, businessman Joel Bigalow gave voice to these reports in a letter to his brothers: “…there are some that assert that it is a preconcerted plan by a lot of villains to meet here and burn the city–for plunder.” An even broader view was that the blaze was set, not by local ruffians, but by a group of international criminals, intent on causing chaos in the world’s large cities. Proponents of this idea point to a similar blaze that rocked Paris earlier that year after the end of the Franco-Prussian War.

And indeed, looting, riots, and general lawlessness ran rampant after the blaze, though whether this was the cause or effect of the disaster in unknown. Fueled by conspiracy theories and a general sense of anxiety and fear among citizens, Mayor Roswell Mason declared martial law in the city and authorized General Philip Sheridan–the General Sheridan of Civil War fame–to gather a group of military men and armed civilians to patrol the streets until order could be restored. This ended when prosecuting attorney (and prominent citizen) Thomas Grosvenor was accidentally shot by an untrained volunteer.

Whatever the cause, the devastation caused by the blaze was swift and extensive. The city’s rebuilding, surprisingly, was even faster. Much of Chicago’s physical infrastructure, including its transportation systems, remained intact, and reconstruction efforts began almost immediately. Planners were able to rebuild the city in a way that incorporated modern considerations, which spurred great economic development and population growth. Less than fifteen years later, the city was home to more than one million people (second only to New York City in terms of population) and hosted the World’s Fair, an economic and tourist attraction boasting an attendance of some 27.5 million.

Today, the Chicago Fire Department training academy is located on the site of the O’Leary property, a wink and a nod to the cow and the lantern that may (or may not) have started it all.