What caused the Great Molasses Flood?

Answer 1

Unseasonably warm January weather in 1919 (one day before the passage of Prohibition) combined with an overfilled storage tank that wasn't strong enough (and hadn't been tested properly) to hold in the pressure caused by thermal expansion and fermentation. The molasses heated up, expanded, and burst the tank.

The resulting flood (approximately 2.3 MILLION gallons of the stuff) killed 21 people, and injured 150. The force of the wave of liquid was enough to break steel girders, and lift a train off its rails.

Interestingly enough, it has been speculated that the failure wouldn't have occurred in the summer, even though you would think there would be more expansion. It's because there was an almost 40 deg F increase in temperature after the tank was filled that's the key physical difference.

Answer 2

The failure of a cast-iron tank full of the stuff at what I think was a rum-manufacturing plant in Boston. Molasses filled the narrow streets instantly and killed a number of people.

It must have been interesting to clean up. Happened in the 1880's, I think. The encyclopedia would tell you.