Boston's Tidal Wave of Molasses

On 15 January 1919, a warm day in Boston's North End, a steel tank holding 2.3 million gallons of molasses split open.

The tank was enormous — roughly 50 feet tall and 90 feet across — and it had been built fast and cheaply for the US Industrial Alcohol Company to hold raw material for industrial alcohol, much of which was being turned into munitions.

The failure was structural and entirely foreseeable. The steel was thinner than the design specified. The tank had never been properly tested with water before being filled. Reports of leaks and rumblings had been dismissed, and neighbourhood children were known to collect syrup from the seams of the tank.

When it went, it sent a wall of molasses up to 25 feet high through the streets at an estimated 35 mph — fast enough to move a firehouse off its foundation, crush rail cars and sweep people off their feet. Molasses is far denser than water, which is why it threw people and buildings around rather than flowing around them.

21 people died. Around 150 were injured.

A class action against the company ended badly for it: the court found the failure was the result of negligence. The verdict took years and produced one of the earliest serious statements of what became a founding principle of modern engineering law — that a company cannot escape liability simply by hiring an engineer and trusting him.

And the smell stayed. For decades residents of the North End claimed that on hot summer days you could still smell molasses in the air. The story goes that it lingered so long that people used it to judge the weather.