1919: A giant molasses tank blows up, sending a wall of thick, sticky syrup through the streets of a Boston neighborhood. The blast and the molasses flood kill 21 people and injure 150.

The Purity Distilling Company built the tank in 1915 on the waterfront of Boston's North End, a populous neighborhood of Italian immigrants just a few blocks from the city's financial and downtown shopping districts. With a diameter of 90 feet and 50 feet high, the iron tank could hold about 2½ million gallons of molasses, ready to be distilled into rum or industrial alcohol.

At least, it could hold the molasses until shortly after noon on Jan. 15, 1919. No one is sure what caused the disaster. Workers and neighbors had complained about the tank leaking for years, so the owner painted it brown to hide the leaks. But the disaster was likely not due to overfilling, because the tank didn't merely give way — it exploded.

The local temperature had risen from 2 degrees above zero to the 40s in a couple of days. It's possible that the rapid heating started a fermentation process, or that newly added warm molasses somehow reacted with colder molasses lower in the tank.

Whatever caused the explosion, 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. Victims couldn't outrun it. It knocked them into buildings and other obstacles, it swept them off their feet, and it pulled them under to drown in a viscous, suffocating, brown death.

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.

More than a hundred separate lawsuits dragged on until 1925, when the U.S. Industrial Alcohol Co., the distillery's owner, finally settled the claims for nearly $1 million (about $12 million in today's money).

One of the strangest industrial accidents ever lingered on, and not just in a few safety improvements. On warm days for decades after, the neighborhood smelled of molasses. And if you listen to old-timers, even today, hot weather brings a vague, sickly sweet smell to the streets of the North End.

Source: Various