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Scientists finally decode the Great Molasses Flood of 1919

Nearly 100 years later, scientists have determined what exactly made Boston's 1919 molasses flood so deadly: the cold. 

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The ruins of tanks containing 2 1/2 million gallons of molasses lie in a heap after the dramatic collapse that hurled trucks against buildings and crumpled houses in the North End of Boston on Jan. 15, 1919.

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After nearly one hundred years, scientists have come up with an explanation for one of history's strangest disasters: the Great Molasses Flood of 1919. 

The deadly 25-foot wave of molasses that flowed through Boston's North End, flattening buildings in its wake, may sound like a scenario in a science fiction movie – but for the 21 people killed and 150 injured in the flood, the nightmarish scene was very real. 

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"Molasses, waist deep, covered the street and swirled and bubbled about the wreckage ... Here and there struggled a form‍ – whether it was animal or human being was impossible to tell," the Boston Post reported at the time. "Only an upheaval, a thrashing about in the sticky mass, showed where any life was ... Horses died like so many flies on sticky fly-paper. The more they struggled, the deeper in the mess they were ensnared. Human beings‍ – men and women‍ – suffered likewise." 

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But how, exactly, did the sticky, sweet substance commonly found in cookies turn so deadly? That's the question researchers set out to answer in a new study presented last week at the annual meeting of the American Physical Society's Division of Fluid Dynamics in Portland, Ore. 

The flood began shortly after 12:40 pm local time on Jan. 15, 1919, when a giant storage tank – 50 feet tall and 90 feet wide – collapsed on Boston's waterfront at the Purity Distilling Co., releasing more than 2.3 million gallons of molasses into Boston's North End. The wave moved through the neighborhood at more than 50 feet per second. 

To explore the physics behind the flood – how it moved so quickly and caused so much damage – researchers combed hundreds of pages of historical accounts and contemporary newspaper articles, studied century-old maps of buildings in the area, and requested historic data from the National Weather Service. 

The scientists also studied the properties of blackstrap molasses, particularly how its rate of flow is affected by temperature. They found that the substance's viscosity, or the degree to which it resists flowing, depends largely on temperature. 

"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.

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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 last year. 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."