On 15 January 1919, it was unusually warm in Boston. People in the North End had their coats open. Children were playing near the elevated railway on Commercial Street.

At around 12:30 p.m., residents heard a deep rumble and a series of sharp cracks, like a machine gun. A huge steel tank on the waterfront, holding about 2.3 million gallons of molasses, had split open.

What came out was a wave.

A wall of syrup

Eyewitnesses described a brown wall several metres high, rolling down the street. It moved fast — estimates often put it at around 35 miles an hour at first — far faster than anyone could run. It knocked buildings off their foundations, buckled a girder of the elevated railway, and swept up people, horses and wagons.

Then came the part that made it so deadly. Molasses doesn't drain like water. As it spread and the January air cooled it, it grew thicker and stickier. People caught in it couldn't swim out. Rescuers couldn't wade in.

Twenty-one people died, including children on their way home from school. Around 150 were injured.

Why was there so much molasses?

Molasses was valuable. It could be fermented and distilled into industrial alcohol, which was used to make munitions. The tank belonged to a subsidiary of the United States Industrial Alcohol Company, and it had been built in a hurry a few years earlier.

The trial

The company first claimed that anarchists had blown up the tank. A long court case, involving thousands of pages of testimony, concluded otherwise: the tank was too weak for its load, built with steel that was too thin and never properly tested by filling it with water.

The company paid out hundreds of thousands of dollars in settlements. More importantly, the disaster helped lead to tougher rules in Massachusetts requiring that building plans be checked and signed by qualified engineers.