Exactly 100 years ago, an enormous wave swept through Boston’s North End, smashing up buildings, taking down horses, and killing 21 people. But this was not the result of extreme weather or a burst dam – it was caused by a viscous, sticky, sugary goo.

On January 15, 1919, North End’s residents heard roars, rumbles, and a crashing sound, followed by an ear-splitting bang. An enormous tank of molasses had burst, releasing 2.3 million gallons of the sticky substance into the streets. The wave apparently reached heights of 7.6 meters (25 feet), traveling at speeds of up to 56 kilometers (35 miles) per hour. Today it is referred to as the Boston Toffee Apple Tsunami, the Great Molasses Flood, or the Boston Molasses Disaster. 

“Molasses, waist deep, covered the street and swirled and bubbled about the wreckage,” the Boston Post reported at the time. “Here and there struggled a form? – ?whether it was animal or human being was impossible to tell. Only an upheaval, a thrashing about in the sticky mass, showed where any life was... Horses died like so many flies on sticky fly-paper. The more they struggled, the deeper in the mess they were ensnared. Human beings? – men and women? – ?suffered likewise.”

The fast-moving molasses took down the girders of a nearby railway structure, knocking a railroad car off the tracks. It swept buildings off their foundations and lifted vehicles off the road. 

About 150 people were injured and 21 were killed. Those lost ranged in age from 10 to 78, but most casualties were workers positioned near to the container. Thanks to the dark sticky nature of molasses, many people turned up to the hospital "looking like toffee apples", according to eye-witness reports. 

The cool January temperatures caused the escaped molasses to be even more viscous than usual, making it easier for people and animals to become stuck. The intense viscosity also made it tricky to release the victims before they suffocated, and made it harder for rescuers to wade to their aid in time.

The explosion took place at the Purity Distilling Company, a subsidiary of United States Industrial Alcohol (USIA), where a 15-meter-tall (50-foot-tall), 27-meter-wide (88-foot-wide) storage tank stood, filled with millions of gallons of molasses. Molasses was fermented to produce industrial alcohol to create munitions during World War I, which had ended just two months before. 

"Once the war ended in November 1918, USIA – aware that the Prohibition amendment was about to be passed in January of 1919 which would ban alcohol sales one year later – retooled their processing plant in East Cambridge, MA and had hoped to produce rum and get it on the shelves before sales were prohibited," Stephen Puleo, historian and author of Dark Tide: The Great Boston Molasses Flood of 1919, told IFLScience. "But the tank collapsed on January 15, 1919, so the company was never able to sell much rum from molasses used in the Boston tank."

So, what led to the tank's demise?

As the molasses was fermenting, it would have released carbon dioxide, which likely upped the pressure within the tank. A change in the weather, as local temperatures suddenly increased, would have exacerbated this. But this pressure change wasn't the only culprit.

Following the incident, an inquiry found that basic safety tests weren’t carried out during the tank’s construction. According to the American Physical Society, the tank used to leak a lot, with some locals siphoning off the leakage for their own personal use. When complaints were made about the leaks, the company simply painted the whole tank the color of molasses, disguising the stains without fixing the tank. A couple of days before the tragedy, the tank had been filled to near capacity, causing it to strain.

A more recent investigation from 2014 also found that the tank was poorly designed. Its steel walls were far thinner than they should have been – half as thick, in fact – and lacking in manganese, which would have made them too brittle.

Attempting to shirk responsibility, USIA claimed that the explosion was the result of a terrorist attack plotted by anarchists. However, when the company was found to be at fault, they eventually paid out a total of $600,000 in settlements.

The cleanup operation took weeks, and for a long time afterward, whenever the weather was hot, the sweet stench of molasses filled the air. The company's negligence led to changes in state laws to ensure that architects and engineers properly inspected and approved plans for big construction projects. 

"Almost all the building construction standards we take for granted today stem from the Great Boston Molasses Flood, including the fact that architects must show their work, that engineers must sign and seal their plans, that building inspectors need to come out and license projects," explained Puleo. "These first started in Boston and Massachusetts and then rippled across the country."

Bizarrely, the Toffee Apple Tsunami is not the only molasses-related disaster in history. Much more recently, in September 2013, about 233,000 gallons of molasses spilled into the waters of Honolulu Harbor in Hawaii, all thanks to a leaky pipe. While no people were harmed, it's estimated that about 26,000 fish and other marine creatures were killed in the incident.