On January 15, 1919—an unusually warm winter day in Boston—patrolman Frank McManus picked up a call box on Commercial Street, contacted his precinct station and began his daily report. Moments later he heard a sound like machine guns and an awful grating. He turned to see a five-story-high metal tank split open, releasing a massive wall of dark amber fluid. Temporarily stunned, McManus turned back to the call box. "Send all available rescue vehicles and personnel immediately," he yelled, "there's a wave of molasses coming down Commercial Street!"

More than 7.5 million liters of molasses surged through Boston's North End at around 55 kilometers per hour in a wave about 7.5 meters high and 50 meters wide at its peak. All that thick syrup ripped apart the cylindrical tank that once held it, throwing slivers of steel and large rivets in all directions. The deluge crushed freight cars, tore Engine 31 firehouse from its foundation and, when it reached an elevated railway on Atlantic Avenue, nearly lifted a train right off the tracks. A chest-deep river of molasses stretched from the base of the tank about 90 meters into the streets. From there, it thinned out into a coating one half to one meter deep. People, horses and dogs caught in the mess struggled to escape, only sinking further.

Ultimately, the disaster killed 21 people and injured another 150. About half the victims were crushed by the wave or by debris or drowned in the molasses the day of the incident. The other half died from injuries and infections in the following weeks. A long ensuing legal battle revealed several possible reasons for the flood. The storage tank had been filled to near capacity on July 13 and the molasses had likely fermented, producing carbon dioxide that raised the pressure inside the cylinder. The courts also faulted the United States Industrial Alcohol Co., which owned the tank, for ignoring numerous signs of the structure's instability over the years, such as frequent leaks.

The Great Molasses Flood of 1919 is both tragic and fantastic. To fully understand this bizarre disaster, we need to examine what makes it unique—its very substance. "The substance itself gives the entire event an unusual, whimsical quality," wrote Stephen Puleo in his book Dark Tide, which recounts the story of McManus and many others who witnessed the calamity.

A wave of molasses does not behave like a wave of water. Molasses is a non-Newtonian fluid, which means that its viscosity depends on the forces applied to it, as measured by shear rate. Consider non-Newtonian fluids such as toothpaste, ketchup and whipped cream. In a stationary bottle, these fluids are thick and goopy and do not shift much if you tilt the container this way and that. When you squeeze or smack the bottle, however, applying stress and increasing the shear rate, the fluids suddenly flow. Because of this physical property, a wave of molasses is even more devastating than a typical tsunami. In 1919 the dense wall of syrup surging from its collapsed tank initially moved fast enough to sweep people up and demolish buildings, only to settle into a more gelatinous state that kept people trapped.

Physics also explains why swimming in molasses is near impossible. One can predict how easily an object or organism will move through a particular medium by calculating the relevant Reynolds number, which in this case takes into account the viscosity and density of the fluid as well as the velocity and size of the object or organism. The higher the Reynolds number, the more likely everything will go along swimmingly.

At least two researchers have directly investigated how people swim in a low Reynolds number environment. Their 2004 study is candidly titled "Will Humans Swim Faster or Slower in Syrup?" Brian Gettelfinger and Edward Cussler, both engineers at the University of Minnesota, asked 16 volunteers—including a few people training for the Olympics—to swim 25 yards (22.5 meters) in a swimming pool filled with plain water and in one filled with water and guar gum, an extract of guar beans used to thicken food. Even though the guar gum doubled the viscosity of water, the volunteers swam equally fast in both pools. The Reynolds number simply did not sink low enough. Gettelfinger and Cussler calculated that in order to challenge human swimmers, they would have needed to increase the viscosity of water 1,000 times.

Depending on the way it is made, molasses is between 5,000 to 10,000 times more viscous than water. The Reynolds number for an adult man in water is around one million; the Reynolds number for the same man in molasses is about 130. To make matters worse, a man immersed in molasses will not get anywhere with the kinds of symmetric swimming strokes that would propel him in water. Each repetitive stroke would only undo what was done before. Pulling his arm towards himself would move molasses away from his head, but reaching up to repeat the stroke would push the molasses back where it was before. He would stay in place, like a gnat trapped in tree sap. Even burly men struggled to tread molasses in the wake of the Boston Molasses Disaster; horses flailed and brayed, straining to keep their heads aloft and snorting to clear their airways.

From the perspective of humans and animals of comparable size, swimming through syrup is a bizarre nightmare scenario; for some of the most abundant life forms on the planet, however, a quagmire of molasses is an everyday reality.

One summer afternoon in 2011 my friend Mara and I were wandering the streets of Boston when we spotted a small plaque near the harbor memorializing the Great Molasses Flood. Around the same time I had been speaking with microbiologists about a newly discovered method of microbial locomotion: a bacterium that strategically flung and detached sticky threads a la Spiderman to slingshot itself through fluid coating a solid surface. The bacterium, Pseudomonas aeruginosa, which lives all over the place—in soil, in people’s houses and in the human body—often encounters fluids and slimes of various kinds, whether mud or mucus. By slingshotting itself around, the researchers proposed, Pseudomonas aeruginosa was taking advantage of “shear thinning”: it used the movement of its body to reduce the viscosity of surrounding fluid. Because bacteria are so tiny, the researchers explained, even a fluid we consider thin—such as plain water—is as thick as molasses to them. Microbes permanently inhabit a low Reynolds number world—a truth made famous by the American physicist Edward Mills Purcell in his 1973 lecture "Life at Low Reynolds Number.” Some bacteria must combat Reynolds numbers as low as 10^-5 (0.00001).

I became fascinated by the idea of microbes battling viscous forces many times greater than those unleashed on Boston in 1919—forces to which most of us are oblivious. So I started researching. I called up my fellow science writer Aatish Bhatia, who had written a fascinating essay called “What it feels like for a sperm,” that I highly recommend. I looked up the transcript of Purcell’s original talk and old papers by pioneers in research on microbial movement, such as Howard Berg. And I searched the research literature for the most recent studies on how microorganisms swim.

Many bacteria and other microorganisms have obvious adaptations to overcome low Reynolds numbers; they row thousands of hairlike projections called cilia or corkscrew their way through fluid with powerful spinning tails known as flagella. Other bacteria and their kin have puzzled researchers by swimming just fine without such external accoutrements. In recent years scientists have revealed how some of these more mysterious microbes get around: a few rely on complex internal motors that ripple the cell surface; one bacterium can turn mucus in the human stomach into a much thinner fluid; and another microbe has, shall we say, a rather kinky way of moving. I describe such adaptations in more detail in a feature article in the August issue of Scientific American—the culmination of my slip-‘n-slide journey into the wonderfully weird world of microbes in molasses.