The 21 people who died in Boston on Jan. 15, 1919, had little warning of the events that were about to occur. According to an article published the next day in The New York Times, the only sound before the disaster was "a dull, muffled roar." That was the noise made by the explosion of a massive tank of molasses owned by the Purity Distilling Company. Moments later, more than 2 million gallons of hot, thick, sticky molasses flooded the surrounding streets, destroying buildings, overturning wagons and trucks, and even knocking an elevated train off its tracks. Witnesses say the wave of molasses reached as high as 30 feet tall and it traveled as fast as 35 miles per hour.

For the people in the surrounding streets and buildings, there was no escape. Twenty-one people died, including three firemen who were killed when their nearby firehouse collapsed. Another 150 people were injured, and several horses were also killed. Police, a local Army battalion, the Red Cross and even the Navy arrived to help the survivors, but rescuers were hampered by the sticky goo that filled the streets. It took four days to find all of the victims, and another two weeks to clean up the molasses mess. Even today, nearly a century later, some people say the neighborhood still smells like molasses on hot summer days.

Coverage of the Great Molasses Flood in the Boston Post. Boston Post [public domain]/Wikimedia Commons

This terrible event has come to be known as the Great Boston Molasses Disaster, one of the most bizarre — and least talked about — tragedies in U.S. history. As Stephen Puleo writes in his excellent book, "Dark Tide: The Great Boston Molasses Flood of 1919," it's possible that the history books rarely take notice of the tragedy because no one "prominent" died that day. "The survivors did not go on to become famous," Puleo writes. "They were mostly immigrants and city workers who returned to their workaday lives, recovered from injuries, and provided for their families."

Why did it turn into a disaster?

So what caused the flood? Purity immediately placed the blame on anarchists, saying the explosion that ripped open the molasses storage tank must have been sabotage. Years of hearings and testimony by hundreds of people revealed otherwise: the tank wasn't built well and it was poorly maintained. It appears that the fermentation process combined with an abnormally warm day caused a buildup of pressure within the tank, more pressure than the roof and walls could contain. The unseasonably warm weather — 41 degrees, up from just 2 degrees that morning — may have also contributed to the high death toll, since more people were on the streets than would have been normal for Boston in January.

More recently, scientists have been digging into the story, looking for clues. Nicole Sharp, an aerospace engineer who runs a popular Tumblr blog, and Jordan Kennedy of Harvard University gathered data from historical records and studied how molasses flows under various conditions, according to New Scientist. They wanted to know if fluid dynamics could help solve the mystery — and they were right.

The true culprit: gravity currents, which come into play when a dense fluid spreads horizontally into a less dense fluid (in this case, molasses into air). It’s similar to how dense cold air will flow through an open door into a warm room, even if there is no wind to drive it. The density of the molasses alone would account for the speed of its initial spread. “Basically, you got bowled over by a tidal wave of molasses,” says Sharp, likening the effect to a sticky-sweet tsunami made of a substance 1.5 times as dense and several thousand times more viscous than water.

Ultimately Purity and its parent company were found to be responsible. The civil lawsuit lasted until 1925. That year the company took a charge of $628,000 against its profits, reflecting settlements and legal costs related to the disaster. That's about $8.3 million in 2013 dollars.

In his book, Puleo points out that the flood — though mostly forgotten today — embodies life in United States during the early 20th century. The tank was built during World War I. Prohibition arrived the following year. The labor movement was growing, dangerous anarchists were active, and the country was dealing with issues related to immigration. "The flood, therefore, was a microcosm of America, a dramatic event that encapsulated something much bigger," he writes, calling it "a lens through which to view the major events that shaped a nation."

Puleo offers a closer look at the Boston Molasses Flood and how it impacted the neighborhood in this video: