“Slow as molasses in January” didn’t apply in Boston on this day in 1919. On January 15, a massive storage tank burst in the North End, unleashing a 25-foot wave of molasses that raced through the streets at an estimated 35 mph. The event, now known as the Great Molasses Flood, killed 21 people, injured 150, and left entire blocks buried in syrup. The tank, owned by the Purity Distilling Company, held more than 2 million gallons of molasses. A sudden temperature swing and structural failures are believed to have caused the explosion just after 12:30 pm. Witnesses reported buildings shifting off their foundations and the ground shaking like a passing train. Cleanup took weeks. Molasses was tracked across the city, into subways, homes, and harbors. Locals famously claimed the North End smelled sweet on hot days for decades after. Today, the site is Langone Park, marked by a small plaque reminding visitors that one of Boston’s most unbelievable moments was also one of its deadliest. ...