Photo: AP

In 1919, a holding tank filled with 2.3 million gallons of molasses ruptured and sent an enormous wave of goop through the streets of Boston. It enveloped and destroyed everything in its path—leaving 21 people dead and around 150 others injured. Until now, no one really knew why it was so deadly, but a team of scientists and students believe they’ve found a solution.

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The New York Times explains:

At a meeting of the American Physical Society this month, a team of scientists and students presented what may be a key piece of the century-old puzzle. They concluded that when a shipment of molasses newly arrived from the Caribbean met the cold winter air of Massachusetts, the conditions were ripe for a calamity to descend upon the city.

By studying the effects of cold weather on molasses, the researchers determined that the disaster was more fatal in the winter than it would have been during a warmer season. The syrup moved quickly enough to cover several blocks within seconds and thickened into a harder goo as it cooled, slowing down the wave but also hindering rescue efforts.

Harvard students used our modern knowledge of fluid dynamics to analyze this century-old case. They started by performing experiments on corn syrup in a walk-in refrigerator. This environment allowed them to simulate how the molasses would’ve behaved in a Boston winter. The students were able to apply the data they gathered to models of a flood over the North End of Boston. They found that the results were a close match to the historical accounts of what occurred in the Great Molasses Flood.

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One thing that the scientific data confirms is the speed at which the molasses moved. People claimed the initial wave was hurtling forward at 35 miles per hour. This led many to believe it was propelled by an explosion. But the Harvard team’s calculations showed that speed would be possible on its own.

Nicole Sharp is an aerospace engineer and science communications expert who acted as an adviser on the project. She tells the Times, “It’s an interesting result and it’s something that wasn’t possible back then. Nobody had worked out those actual equations until decades after the accident.”

When the molasses was delivered two days before the spill, it was heated in order to make it easier to transfer. Sharp says that the molasses was probably still four of five degrees warmer than the outside air when the disaster struck. The students determined that as the molasses flowed around victims it quickly trapped them as its viscosity raised in the cool air.

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The student’s investigation grew out of Professor Shmuel M. Rubinstein’s class Introduction to Fluid Dynamics. He says that he instructed students to “choose an interesting project and make an appealing video.” You can see the video over on the Times full story, I highly recommend it. The video shows a slow-mo flood simulation done with tiny models. It has an old fashioned science fair quality to it.

Rubinstein and Sharp are now considering building a whole course around this particular case. It could be a sort of forensic physics class in which many areas of study are applied to various factors that contributed to that tragic day. For example, there’s still no definitive answer for why the tank burst that day. Rubinstein says it’s “a beautiful story for teaching.”

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I’d also recommend checking out the vintage clip from the New York Times coverage that day. It has that incredible newspaperman prose that you just don’t see anymore.