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.