The trouble, it seems, was the tank in which the molasses was stored, despite initial claims from the company responsible for the disaster – they blamed anarchists. The tank was built in 1915 under the supervision of Arthur Jell, who lacked any background in engineering or architecture, according to a recent report looking back on the event, which came to be known as the Great Molasses Flood in Boston’s North End.

Standing 50 feet tall, 90 feet in diameter and holding 2.5 million gallons of molasses, the tank was notoriously leaky and made from thin, brittle steel. Purity Distilling Company, which owned the tank and the processing plant on the same site, decided to simply paint the tank a dark brown to cover up the leaks instead of repairing it sufficiently. As one study indicated, “The tank needed to be an engineering marvel to hold all that weight, but the company never even consulted an engineer on the project. Basically, it threw up a gigantic tank as quickly and cheaply as possible, skimped on inspections and safety tests and hoped for the best.”

A report published in 2015 stated the walls were “at least 50 percent too thin” to hold the volume of molasses contained within the tank, the Boston Globe reported.

When the tank exploded and released the wave, 21 people were killed and more than 150 others injured. The power and weight of the molasses knocked buildings off their foundations and trapped people inside. Those unlucky enough to be directly in front of the wave as it traveled were suffocated under the sweet substance. The Boston Globe reports at least one person was swept into Boston Harbor by the wave.

There were other factors contributing to the speed at which the molasses moved. For starters, just two days before the explosion, warm molasses was added to the tank, filling it to capacity for only the fourth time since it was constructed. The molasses was at a temperature of about 10-20 ℃, or 50-68 ℉, when it arrived, while the air temperature was 5 ℃, or 41 ℉. As the molasses flowed down the streets, it cooled and thickened, making it slower.

And how do you even begin to clean up that much molasses? Fresh water wasn’t very effective, though that was the tool of choice initially for local firefighters. Instead, they used salt water and flushed the waste into the city’s sewers. Even then, it took more than 80,000 man-hours to rid the North End of the mess.

The legal proceedings that followed were the longest in the city’s history to date, writes Chuck Lyons over at History Today. He details some of the victims, including 10-year-old Pasquale Iantosca, who was crushed by a railroad car, and railroad clerk Walter Merrithew, who was pinned against the wall of a freight shed and watched a horse drown while his feet were three feet above the ground.

“Bodies were often so covered by a brown glaze that they could not be seen. The body of truck driver Flamino Gallerini was taken from the water underneath the railroad fright houses eleven days after the tank burst, and almost four months after that a final body, that of Cesare Nicolo, was pulled from the water under the Commercial Wharf,” Lyons writes.

A year after the flood, 119 lawsuits had been filed against United States Industrial Alcohol, Purity Distilling Company’s parent company. The first hearing took place on August 9, 1920; three years, 921 witnesses and more than 25,000 pages of transcript later, it was over, the longest and most expensive civil suit in Boston’s history. Hugh W. Ogden, the judge in the case, gave his verdict on April 28, 1925, holding the company responsible for the accident and dismissing their claim that Italian anarchists blew up the tank. “I believe and find that the high primary stresses, the low factor of safety, and the secondary stresses, in combination, were responsible for the failure of the tank,” he ruled.

The property damage was valued at around $100 million in today’s US dollars; each of the 21 families that filed suit against United States Industrial Alcohol received $7,000.