Despite the force of the rupture, initial damage was limited to the tank's adjacent buildings. The two-and-a-half million gallon column of molasses, however, caused a considerable catastrophe as it spread itself out into the North End neighborhood.
Bystanders were swept up in the smothering goo--tossed and rolled through the thick sludge. The migrating wave of brown syrup pushed buildings off their foundations and overturned wagons, carts, horses, and motorcars. It broke the girders of an elevated rail track, and tossed a train from its tracks. Within minutes several blocks of Boston's streets were buried in struggling victims, rubble of ruined buildings, assorted wreckage, and 2-3 feet of sweet, tacky goo.
Rescue efforts began immediately, but most who ventured in became mired in the mess and soon required rescuing themselves. Terrified survivors were seen running away from the chaotic scene covered from head to toe in dark brown molasses. The USS Nantucket was anchored at the Playground Pier a few block away, and Lieutenant Commander H. J. Copeland sent over a hundred of his able-bodied sailors to lend assistance. Police officers, military personnel, and Red Cross nurses slogged through the knee-deep syrup all night long, searching for sticky victims.
The exact cause of the explosion was never determined definitively, but it is generally attributed to high pressure and a defects in the tank construction. According to some reports, the Purity Distilling Company had neglected to pressure-test the vessel prior to filling it for the first time; so upon its first load of molasses, the outside of the tank was striped with molasses leaks. Rather than paying for repairs, Purity opted to paint the tank brown. On that fateful day in 1919, pressure increases due to fermentation of the molasses and unseasonably warm temperatures put too much strain on these fractures, causing an energetic rupture.
Although its been almost a century since the flood, they say that on a hot day the streets in some parts of Boston still bleed molasses.