On January 15, 1919, Boston's 2.3 million gallon molasses flood killed 21 people

The great Boston molasses flood (wiki):

On January 15, 1919, a tank containing 2.3 million gallons of molasses weighing an estimated 26 million pounds burst open, unleashing a sticky flood onto Boston's North End. The 25-foot high wave of goo oozed over the streets at 35 miles per hour, crushing buildings in its wake and killing 21 people.

The wave broke steel girders of the Boston Elevated Railway, almost swept a train off its tracks, knocked buildings off their foundations, and toppled electrical poles, the wires hissing and sparking as they fell into the brown flood. The Boston Globe reported that people 'were picked up and hurled many feet'. Rivets popping from the tank scourged the neighborhood like machine gun bullets, and a small boat was found slammed through a wooden fence like an artillery shell. By the time it passed, the wave had killed 21 people, injured 150, and caused damage worth $100 million in today's money. All caused by molasses.

Wired:

The tank gave out a dull roar, and then its two sides flew outward with a mighty blast. One huge piece knocked out the support of an elevated railway, buckling the tracks. An engineer stopped his train just in time to avoid an even worse disaster. Fragments of metal landed 200 feet away.

Besides sending shrapnel whizzing through the air, the explosion flattened people, horses and buildings with a huge shockwave. As some tried to get to their feet, the sudden vacuum where the tank once was created a reverse shockwave, sucking air in and knocking people, animals and vehicles around once more, and shaking homes off their foundations.

That was just the first few seconds. The real terror was about to begin.

The tank had been filled to near capacity, and 2.3 million gallons of thick, heavy, odorous molasses formed a sticky tsunami that started at 25 or 30 feet high and coursed through the streets at 35 mph...

When it was over, more than a score had died, and seven or eight times that number suffered injuries. The mess took months to clean up, and the legal issues even longer.

It was the height of the post-World War I Red Scare, and the distillery blamed anarchists, who they said knew the molasses was intended for alcohol to make military ammunition. The victims and their survivors blamed the distillery for faulty construction and unsafe operation.

At the time, molasses was a standard sweetener in the United States, used in cooking and in fermentation to make ethanol, which in turn could be made into a liquor used as an ingredient in munitions manufacture, an aspect of the business that had been booming during the First World War.

A map of the flood was printed in the paper in the days following the disaster.
 —The Boston Globe Archives

The tank itself was just over three years old. It was constructed of large curved steel plates, seven vertical rows of them overlapping horizontally and held together with rows of rivets, the whole set into a concrete base. It was perfectly located for USIA, just 200 ft from the harbor and ships that brought molasses from Cuba, and near the railroad tracks that would move the molasses from storage.

Yet the five-story storage facility was never properly tested - by filling it with water - because a shipload of molasses was due only days after the completion of the tank in December 1915.

Excerpts from an article at History Today:

On January 15, 1919 at around 12.30 pm, lunchtime for many workers, the tank broke. Buildings of the nearby Northend Paving Yard were instantly reduced to kindling as the molasses cascaded out. The three story Engine 31 Fire House was torn from its foundations, trapping three firefighters who fought to keep their heads above the rising tide. A piece of the tank was blown into the elevated railway tracks, breaking girders and almost forcing a northbound train off its tracks.

The entire waterfront area was leveled and rails from the overhead railway dangled like Christmas tinsel.

Here's a brief documentary:

First on the scene were 116 sailors from the lightship USS Nantucket that was docked nearby. They were soon joined by Boston police, Red Cross workers and army personnel. When Suffolk County medical examiner George Magrath arrived, several bodies had already been pulled from the molasses. He said they looked 'as though covered in heavy oil skins ... eyes and ears, mouths and noses filled'. A makeshift hospital was set up at Haymarket Relief Station about half a mile from the waterfront, and volunteers removed molasses from victims' noses and mouths so they could breathe.

The rescue continued for days, and almost four months later a final body was pulled from the water under the Commercial Wharf.

Harvard students re-enacted the Molasses Flood
 using a small scale model, corn syrup, and a walk-in refrigerator.

Via Popular Science, researchers at Harvard have gained new insight into the disaster by studying historical accounts of the accident, century-old maps, and weather data, and building their own mini molasses flood inside a walk-in refrigerator. Using corn syrup as a stand-in for molasses at this smaller scale, they studied the goop's flow properties at wintery temperatures.

That January's relatively balmy, 40-degree Fahrenheit weather may have increased pressure as carbon dioxide built up inside. Before the rupture, the molasses inside the tank was likely 5 degrees Celsius warmer than the surrounding air, thanks to a fresh shipment of syrup from the Caribbean. The higher temperature may have helped the molasses spread over the Boston waterfront at such an alarming pace. But once exposed to the nearly freezing winter temperatures outside, the goo cooled and became much thicker and stickier. 

The higher viscosity may have trapped people caught in the flow, and likely hampered rescue and cleanup efforts, according to the researchers. 

History Today has more on the ensuing lawsuits.

The tank was never rebuilt. The site where it stood is now a public park with bocce (Italian boules) courts and Little League baseball fields, slides and swings. All that remains of that terrible day 90 years ago is a small plaque at the entrance of the recreational complex. Yet local residents insist a faint smell lingers to this day. They say that on warm summer days the air is still tinged with the sweet, cloying scent of molasses.

More at Wired, History Today, Popular Science, Boston.com, and Mental Floss, plus Scientific American has much more on the fluid dynamics involved. The Atlantic has a larger set of photos, which you can view full screen.