On January 15, 1919, a large tank filled with molasses exploded in Boston, resulting in 21 fatalities and 150 injuries. The 15-meter-high tank contained an estimated 8,700 cubic meters of molasses, which is a dense liquid used in ethanol production. Immediately after the explosion, a massive wave of molasses moved through the city streets. Witnesses described hearing a powerful rumble or a loud bang, with rivets from the tank detaching rapidly.
The molasses wave reached a height of 8 meters and moved at approximately 56 kilometers per hour. Its density and volume generated sufficient force to bend steel girders of the Boston Elevated Railway. Houses in its path sustained significant damage. Many individuals caught in the wave found it difficult to swim due to the molasses's thickness. After the molasses settled, it became even thicker, trapping victims. Some victims were not located for months, having been swept into the harbor.
The aftermath involved extensive cleanup, with streets washed with saltwater and covered in sand. The molasses's stickiness was widely distributed across the city by pedestrian movement. For months following the incident, Boston Harbor appeared brown as molasses settled on the seabed.
The precise cause of the explosion has not been definitively determined. One theory suggests that the fermentation of the molasses led to a substantial accumulation of carbon dioxide within the tank. Another theory proposes that rapid temperature fluctuations contributed to the blast. Warmer molasses had been pumped into the tank containing cooler molasses earlier on the day of the event. The ambient temperature also increased significantly that day, from below freezing to 4 degrees Celsius.
More than 100 lawsuits were filed against the United States Industrial Alcohol Company. The company paid approximately $27 million in settlements. Locals reported smelling molasses in Boston on warm days for decades after the event.