Here’s what you’ll learn when you read this story:

• Early in 1919, a shoddily constructed industrial tank, under immense pressure from gas produced by hot molasses poured onto cold molasses, could take no more.
• The explosion that ensued took 21 lives, injured many more victims, and just about flattened several blocks in the North End of Boston. Cleaning up the sticky mess was a waking nightmare.
• After victims and their families filed an enormous class action lawsuit, it took six years before the company behind the disaster was held accountable.

On the afternoon on January 15, 1919, a dark wave suddenly swept into the North End of Boston. But this was no typical nor’easter encroaching in the dead of winter. It was the sickly-sweet devastation that issued from an exploding tank of molasses. The explosion flooded Commercial Street, tearing down buildings and crushing cars, wagons, and everything else in its path—and taking a toll in human lives.

There had been previous signs of trouble. The tank’s construction was overseen several years previously by a middle manager of the United States Industrial Alcohol company, or USIA, whose subsidiary distilled molasses into industrial alcohol. Arthur P. Jell had no engineering experience and succumbed to the pressure of completing the tank on a tight deadline. He cut every corner possible to compensate. He brushed off consulting with actual engineers and skipped arranging materials inspections, but he arguably showed the most neglect in not bothering to test the tank before using it. He was out of time when a shipment of molasses arrived from Cuba on New Year’s Eve. The unfinished tank was promptly loaded with seven hundred thousand gallons of molasses, and it began to crack almost immediately.

Without much to hold it back, molasses oozed through unsealed seams and into the tin pails of delighted children who skipped home with what was essentially free candy, as Stephen Puleo recalled in his book Dark Tide: The Great Boston Molasses Flood of 1919. The North End was a congested neighborhood. There were complaints that the tank, visibly falling apart, was uncomfortably close to residential buildings. Some employees of USIA were concerned. Isaac Gonzalez, the onsite manager of the tank, resigned after raising these issues to management, which responded by painting it brown to hide the sticky globs.

The final straw was an incoming shipment of 1.3 million gallons of molasses that was heated so it would pour more easily. The hot molasses collided with cold molasses already in the tank, causing rapid fermentation that produced gas. Already riddled with cracks, the flimsy tank could no longer take such pressure.

With a menacing hiss, the tank burst into shards of metal and unleashed a tsunami wave of molasses on Boston. As an outpouring of viscous liquid coursed through the streets at speeds of up to 35 mph, metal scraps and loose rivets from the tank turned into instant projectiles. Everything seemed to vanish into a black chasm as the sludge began to swallow the North End. Waves of molasses up to 25 feet high charged in every possible direction, ruthlessly taking down everything in sight. Electrical wires were ripped from their poles and a railroad collapsed under the weight of the molasses tide. People in the street were swept up or crushed.

“Molasses, waist deep, covered the street and swirled and bubbled about the wreckage,” wrote one Boston Post reporter cited in Dark Tide. “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.”

Chaos erupted. Rescue teams frantically tried to dislodge anyone who had become stuck, but it was difficult to dislodge many of the victims from the viscous mass, and not all could be saved. Several young children were smothered. Many survivors had breathed in molasses and needed their airways cleared, suffered broken bones from the brutal force, or ended up with concussions. Mass destruction was left in the wake of the flood once the molasses cooled and slowed down in the midwinter chill. A large section of the North End had been flattened, and every street near the burst tank was coated in a thick layer of the sugary stuff. What was once part of Boston’s waterfront had been leveled. The disaster had taken 21 lives and injured another 150.

Submerged in molasses, the North End struggled to find its way out. Hose water would not wash it away. Some of the molasses had hardened enough to cut through and throw into the harbor. Molasses that wasn’t in solid form was attacked with seawater, which relief workers pumped into the streets after they found it was the only thing that could actually dissolve the morass. Meanwhile, legal claim after legal claim was filed. USIA denied negligence, blaming the disaster on anarchist sabotage. Eventually, 119 claims were combined into the largest class-action lawsuit in Massachusetts history at that time. It took nearly six years for USIA to finally be held accountable for the tank’s fatally flawed construction.

“These were ordinary people caught up in a tragic event, one that ushered in a year of turmoil in the United States,” Puleo wrote, “and later, served as a catalyst for government to impose new safety regulations on industry to protect the public.”