The Great Molasses Flood

Shortly after 12:30 p.m. on Wednesday, January 15, 1919, people along Boston’s North End waterfront heard a deep rumble.

Then the ground seemed to shake.

From a massive storage tank on Commercial Street came the sounds of tearing steel and snapping rivets. Within moments, the tank split apart, and more than two million gallons of molasses began rushing into the neighborhood.

The resulting wave was anything but slow.

Contemporary accounts and later reconstructions vary on its exact height, but estimates commonly place it at roughly 15 feet or more in places. At its fastest, the molasses may have surged through the streets at approximately 35 miles per hour—a remarkable figure that later fluid-dynamics research found physically plausible. The Boston Public Library’s account of the disaster likewise documents contemporary estimates of a 15-foot wave moving at roughly 35 miles per hour. On that January afternoon, the expression “slow as molasses” suddenly had an exception.

The source was an enormous steel tank belonging to the United States Industrial Alcohol Company, or USIA. Built in 1915, it stood about 50 feet tall and 90 feet across, looming over a densely populated section of Boston’s waterfront. USIA stored molasses there before converting it into alcohol; during World War I, industrial alcohol was in heavy demand for the manufacture of munitions and other products.

Shortly before the disaster, a shipment from Puerto Rico had arrived and filled the tank to roughly 2.3 million gallons—not far from its capacity.

The tank had been troubling almost from the beginning.

Its construction had been rushed, and the company official who supervised the project had no engineering background. Before being put into service, the tank was never subjected to a proper full-scale water test. Once it was filled with molasses, it leaked.

The leaks became part of neighborhood life. Children reportedly gathered around the tank to collect the sweet liquid that seeped through its seams. What looked almost harmless was, in reality, a warning.

There were others. Residents heard groans and vibrations when the tank was heavily loaded. The steel strained under the enormous weight inside it. Yet the structure remained in service.

Modern engineering analysis has made the picture even worse. Investigators concluded decades later that the tank’s steel walls were too thin and that the steel itself was unusually brittle in cold temperatures. Weaknesses around the riveted joints also appear to have contributed to the failure.

The disaster, in other words, was not the product of one freak circumstance. The tank had serious structural vulnerabilities built into it, and the neighborhood had been living beside them for years.

On January 15, they finally gave way.

The force of the escaping molasses was extraordinary. Buildings were smashed. Vehicles were swept aside. The nearby elevated railway was damaged. A firehouse was knocked from its foundation.

People and horses caught in the flood faced two different dangers at once. At first, the molasses moved with tremendous speed and force. Then, as it spread through the streets and cooled in the January air, it became thicker and more difficult to escape.

The Boston Post, in an account reproduced by the Boston Public Library, described horses dying “like so many flies on sticky fly paper.”

Rescuers entered an almost surreal landscape. Streets were filled with molasses, wreckage was scattered across the neighborhood, and nearly everything the flood had touched was coated in a dark, sticky film. The colder the molasses became, the more viscous it grew, making it increasingly difficult for trapped victims to free themselves—or for rescuers to reach them.

Twenty-one people ultimately died and roughly 150 were injured.

In the aftermath, USIA offered another explanation. The company argued that the tank had been bombed, pointing to the very real threat of anarchist violence in the United States at the time. But investigators found no persuasive evidence of sabotage.

Instead, the evidence led back to the tank itself: rushed construction, inadequate engineering oversight, structural weaknesses, persistent leaks, and warning signs that had gone unaddressed.

The disaster eventually became an important lesson in industrial accountability. In the years that followed, Boston and other jurisdictions strengthened requirements for engineering review, construction permits, and inspection of large structures. The flood helped reinforce a principle that seems obvious today but was far less firmly established in the early twentieth century: companies building potentially dangerous structures had an obligation to demonstrate that those structures were safe.

The Great Molasses Flood is remembered partly because it sounds almost absurd—a Boston neighborhood swallowed by a wave of syrup. More than a century later, the strangeness of the event can make it easy to forget what it actually was.

It was an industrial disaster.

Twenty-one people died not because molasses behaved strangely, but because an enormous structure had been built and operated without adequate regard for whether it was safe.

The molasses simply exposed the consequences.