“As slow as molasses in January.” There was one memorable exception to that truism. And it was a deadly one.

Forty minutes past noon on 15 January 1919, a giant wave of molasses raced through Boston. The unseasonably warm temperature (46 degrees) was the final stress needed to cause a gigantic, filled-to-capacity tank to burst. 2,320,000 gallons (14,000 tons) of molasses swept through the streets, causing death and destruction.

Eyewitness reports tell of a “30-foot wall of goo” that smashed buildings and tossed horses, wagons and pool tables about as if they were nothing. Twenty-one people were killed by the brown tidal wave, and 150 more were injured. The chaos and destruction were amplified — and rescue efforts were hampered — by the stickiness of the molasses. Those persons attempting to aid others all too often found themselves mired fast in the goo.

The day after the disaster, the New York Times reported:

A dull, muffled roar gave but an instant’s warning before the top of the tank was blown into the air. The circular wall broke into two great segments of sheet iron which were pulled in opposite directions. Two million gallons of molasses rushed over the streets and converted into a sticky mass the wreckage of several small buildings which had been smashed by the force of the explosion. The greatest mortality apparently occurred in one of the city buildings where a score of municipal employees were eating their lunch. The building was demolished and the wreckage was hurled fifty yards. The other city building, which had an office on the ground floor and a tenement above, was similarly torn from its foundations. One of the sections of the tank wall fell on the firehouse which was nearby. The building was crushed and three firemen were buried in the ruins.

In November 2016, new findings about the Great Molasses Flood were presented at the American Physical Society’s Division of Fluid Dynamics in Portland:

“Temperatures dipped just below freezing the night following the accident,” aerospace engineer and fluid dynamicist Nicole Sharp, lead author of the study, told Live Science. “Based on our data, it’s possible the viscosity of the molasses increased by a factor of four or more due to that drop in temperature. That does not sound like such a big difference, but the high viscosity of the molasses was a major factor for rescue work.” If the tank had collapsed in warmer weather, the molasses would have been more like honey than tar, and would have “flowed farther, but also thinner,” reducing the number of people who became fatally stuck, said Shmuel M. Rubinstein, a Harvard University professor whose students investigated the disaster, according to The New York Times. About half of the victims of the flood, he added, “died basically because they were stuck.” The original cause of the deadly disaster, which has remained a mystery for the past century, was explored in another study published [in 2015]. Ronald Mayville, a senior structural and metallurgical engineer with Simpson, Gumpertz & Heger in Waltham, Mass., concluded that several design flaws had contributed to the tank’s collapse. The walls of the tank, Mr. Mayville argued, were too thin to hold 2.3 million gallons of molasses and made from a low-manganese steel susceptible to fracture – the same type of steel, coincidentally, used on the Titanic. “The steel conformed to the standards of the time,” Mayville told the Boston Globe. “But now it’s known you need to have a higher ratio.”

Boston is not a city that forgets anything easily. There are those who claim that on a hot summer day in the North End, you can still smell the molasses.