Slow as molasses isn’t just a saying—the byproduct of sugar production is usually sticky and viscous, even at room temperature. So historians and scientists have long been stumped by Boston's 1919 Great Molasses Flood.

Ethan Trex at Mental Floss reports that on January 15, 1919, a massive molasses holding tank in Boston’s north end owned by the Purity Distilling Company, which used the treacle to produce alcohol, ripped open. A 2.3-million-gallon, 26-million-pound wave of the sticky stuff rolled down Commercial Street as fast as 35 miles per hour. It smashed houses and buildings and knocked a firehouse off its foundation. In the end, the sticky tsunami killed 21 people and severely injured 150. By one estimate, Trex reports, it caused $100 million in damage in today’s dollars.

Though an anarchist terrorist attack was first blamed for the calamity, investigators soon pointed at the holding tank’s shoddy construction. But the question has remained, why did the molasses explode as a wave and not just slowly drip out of the tank? A group of students at Harvard investigated the event and presented their conclusions at recent meeting of the American Physical Society.

“I’m originally from Arkansas, where we have an old expression: ‘Slow as molasses in January,”​ Nicole Sharp, aerospace engineer and science communicator who led the group, tells William Kole at the Associated Press. “Oddly enough, that’s exactly what we’re dealing with here, except that this molasses wasn’t slow.”

Sharp and her team researched historical accounts of the incident as well as National Weather Service data to understand weather conditions in Boston that day. They then performed experiments in a walk-in refrigerator with corn syrup, which has a similar consistency to molasses, to understand how it flows at different temperatures and to model the molasses incident.

According to Erin McCann at The New York Times, the Distillery received a shipment of molasses from Puerto Rico two days before the rupture. The researchers believe that the massive amount of molasses did not have time to completely cool down from its trip from the Caribbean and was likely seven to nine degrees Fahrenheit warmer than the chilly Boston air.

When the tank gave way, the warm molasses spilled out in a huge wave, but it cooled very quickly as it hit the cold air, causing it to become thick and sticky. If it had burst during the summer, the researchers say, the molasses would have likely flowed farther and been much thinner. It would have been a mess, but probably not a similarly fatal disaster.

After the initially 35 mile-per-hour tsunami, which has been reported to be between 15 and 40 feet high, the molasses cooled and slowly crept around the neighborhood. Sharp explains to Carol Off for CBC radio:

It seems like that would be the point where it’s not as dangerous any more. It's not crashing through buildings after that first minute. It's just kind of oozing instead. But it turns out that, because it was cold and because the molasses was cooling, that, if anything, that made the molasses more dangerous. Now people who have been knocked down by that initial wave who may have been pinned in wreckage are trapped in places where they have to try to keep this molasses away from their mouth and nose so they can breathe while people are trying to come and get them. That extra cold makes the molasses easily four or more times as viscous as before and that makes it much harder to fight.

It’s not the first time researchers have looked into the Great Molasses Flood. Last year, an engineer who researched the construction of the holding tank concluded that it was 50 percent too thin to hold 2.3 million gallons of molasses, reports Peter Schworm at The Boston Globe. Though Purity Distilling was found liable for the accident after a three-year trial, and poor construction was to blame, until this study nobody knew exactly why the tank burst.

Stephen Puleo, author of Dark Tide: The Great Boston Molasses Flood of 1919 tells Schworm that the tank was a problem from the beginning and was never properly inspected. The company actually changed the color of the tank from blue to brown-red to cover its leaks.