Back in January 1919, a devastating event took place in Boston's North End neighbourhood: a vat of molasses at the Purity Distilling Company erupted, sending a 7.6-metre-tall (25-foot) wave of treacle-like sweetener into the streets outside.

This bizarre event – known as the Great Molasses Flood of 1919 – saw over 8.7 million litres (2.3 million gallons) of the sticky substance dumped into the streets, where it travelled at roughly 56 kilometres per hour (35 mph), reportedly killing 21 people and wounding 150 more.

The strength of this molasses wave – which was, at the time, America's favourite sweetener and a key ingredient for rum production – was so strong, it reportedly ripped a fire station from its foundations, smashed several freight cars, and even came close to derailing a train.

Despite the damage, no one had been able to figure out how or why the wave became so powerful, but a team of students from Harvard University are finally uncovering clues as to what happened that chilly January day.

"It's a ridiculous thing to imagine, a tsunami of molasses drowning the North End of Boston, but then you look at the pictures," the students' professor, Shmuel Rubinstein, told Erin McCann at The New York Times.

Here's what he means:

BostonMolassesDisasterBPL/WikiMedia Commons

The team examined the physics of how cold weather acts on molasses, finding that the hot syrup moved in a wave down a few city blocks before firming up.

This means the entire area affected by molasses likely turned into a hardened mass, making rescue efforts incredibly dangerous.

Just imagine getting knocked down by a wave of liquid sugar, and then being unable to free yourself as it turned solid – or at least more firm – around your beaten frame. It sounds like something out of a nightmare, but the team thinks that's actually what happened.

The researchers also wanted to verify how fast the wave travelled, and how it could be possible to hit the speeds it reportedly did. 

"The historical record says that the initial wave of molasses moved at 35 miles per hour [56 k/h], which seems incredibly fast," the team's adviser Nicole Sharp, an aerospace engineer who runs the website FYFD, told The New York Times.

"At the time, people thought there must have been an explosion in the tank, initially, to cause the molasses to move that fast."

But throughout their experiment, the team found that heated molasses could move that fast even without an explosion or some kind of external force acted upon it.

They came to this conclusion by studying how corn syrup – a sweetener that they used as a stand-in for molasses – reacted to different temperatures inside a walk-in refrigerator.

They then recreated a scale model of the event that simulated how molasses at various temperatures could spread through the streets of Boston, finding that their model accurately predicted the same results that occurred in real life.

Most importantly, this model shows that the molasses could actually travel up the speeds reported during the event, and that the molasses likely spread fast and suddenly hardened up.

The team suspects that the molasses was likely shipped to Boston from a tropical climate in the Caribbean, keeping its internal temperature rather warm inside the vat.

Once this warm mixture broke free, the air around it quickly cooled the molasses, causing it to become a tar-like substance.

The team presented their findings at a recent meeting of the American Physical Society, but they've yet to be peer-reviewed, so we'll have to wait for independent analysis before we can say that this is the definitive break-down of the event.

But further study could help answer the final question surrounding the Great Molasses Flood of 1919: how did the molasses break free from its vat? Did the vat fall and break? Was it punctured somehow? Your guess is as good as any.

The team suggests that the vat was made with structural flaws, such as the steel being too brittle or thin for the job it was designed for. But that still doesn't explain how it failed in such a colossal way, and launched a tidal wave of molasses over an entire neighbourhood.

With further study, we might finally know the full story, but for now, that part will remain a mystery.