21 people were killed and a further 150 were injured after 2.3 million gallons of molasses swept through North End in Boston, Massachusetts after a storage tank collapsed.
The 50 ft (15 m) tall tank on Commercial Street had been built in 1915 by the Purity Distilling Company to store molasses that were then fermented to produce industrial alcohol for liquor and munitions manufacturing. Demand for the latter had soared following the outbreak of the First World War, and the company had rushed to take advantage. The storage tank was built quickly and was already known to leak, since the metal used for its construction was poor quality and only half as thick as it needed to be to hold the intended quantity of molasses.
A warm shipment of molasses was added to the tank two days before the disaster, decreasing the overall viscosity of the contents and enabling it to flow faster while also increasing the pressure on the tank walls. It has since been suggested that the dramatic increase in air temperate from -17 to 5.0 °C over the course of the previous day may have also increased the speed of fermentation within the tank, adding further pressure from carbon dioxide.
The tank collapsed shortly after noon, and witnesses later recalled feeling the ground shake and hearing a variety of loud crashes followed by a sound similar to gunfire as the rivets shot out of the tank. A wave of molasses measuring over 15 feet swept down the street at 35 miles an hour, smashing houses and seriously damaging the support girders of a nearby elevated train track.
Many of the victims were either crushed by the force of the wave or drowned in the sticky syrup. The youngest victims were just ten years old.