First, let's get our heads around Boston's North End in the early 20th century. It was one of the most crowded residential neighborhoods in the whole world in 1919. 

40,000 people in a little over a square mile - four times today's population. And that’s just the residents. It was also one of the country’s biggest commercial ports, said Steve Puleo, author of Dark Tide.
"The tank was really plunked down in one of the busiest neighborhoods in all of America," Puleo said.

The tank in question was 50 feet tall, 90 feet in diameter. Picture two Boston triple decker homes, pushed together. Inside was 2.3 million gallons of molasses.

And why did we need that much molasses? It was used to make industrial alchohol used to make dyes and lacquers, said Puleo. But when WWI broke out in 1914, another use arose.

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"That industrial alcohol was further processed and used in the production of munitions. High explosives, TNT, nitroglycerin, for the war effort," said Puleo.
So, the tank was rushed to completion in 1915 and corners were cut. The steel wasn’t thick enough, and the tank was never properly tested.

"From the very first day the tank leaked," Puleo said. "Every time it was filled it buckled, it groaned."
That it happened was perhaps inevitable. But the havoc it wreaked was unimaginable. Just after noon, on January 15, 1919, the tank – filled to capacity - collapsed. Officer Frank McManus’s call to police headquarters moments later said it all.
"Send all available rescue personnel immediately, there’s a wave of Molasses coming down Commercial Street."
A 25-foot high tidal wave of brown, viscous molasses speeding through the North End at 35 miles per hour, consuming and destroying everything in it’s path.
"It killed horses, it killed people, it killed domestic animals. Just obliterated a blacksmith shop, a carpentry shop, knocked the firehouse on Boston Harbor off it’s foundation, trapped firefighters underneath, takes out the main tressel of the Boston passenger rail line that ran from South Station to North Station," said Puleo. "Firefighters needed to lay ladders across the molasses and crawl out to pull victims out."
The whole event lasted just 15 to 20 minutes, and left the neighborhood in chaotic shambles. Sailors and firefighter scrambled furiously to clear debris and find victims in the coagulating gunk.
All told, 21 people were killed - including two 10-year children. Some 150 more were injured everything from crushed pelvises to broken backs. The cleanup took months--  and millions of gallons of saltwater to cut the molasses and wash it into the harbor, which was stained brown for weeks on end. And the story doesn’t end there.

The molasses flood spawns a huge civil lawsuit afterwards. One of the first class action suits of its kind, the case stretched on for three years. In a landmark decision, the judge found for the 114 plaintiffs– and ordered the company that owned the tank to make amends.
"It is one of the first decisions against a large U.S. Corporation at the time. Goes a long way toward changing building and construction standards. First in Boston, and then across the country. Almost all of that is a direct result of the Great Boston Molasses Flood of 1919."
The Great Boston Molasses Flood of 1919. One of the most usual – and tragic – chapters in the city’s long history – endured by the residents and workers in the North End, 96 years ago this week.