Slow as molasses? Not when 2.3 million gallons of black treacle engulfs the North End of Boston at 35 miles per hour.

The Great Molasses Flood of 1919 — one of Boston's most peculiar disasters — killed 21 people, injured 150 others and flattened buildings when a giant storage tank ruptured.

Nearly 100 years later, Harvard University researchers finally think they know why the tsunami, which reportedly measured 25 feet tall, claimed so many lives.

A team of fluid dynamics experts concluded that cold temperatures quickly thickened the syrupy mess, which might have claimed few if any lives had it occurred in spring, summer or fall.

The Great Molasses Flood killed 21 people, injured 150 others and flattened buildings when a giant storage tank ruptured. Pictured, a file photo of the destruction on January 15, 1919

Team leader Nicole Sharp said she hopes the findings — presented last week at a conference of the American Physical Society — will shed new light 'on the physics of a fascinating and surreal historical event.'

Two days before the disaster, the molasses arrived in the harbor from the Caribbean, and heated to reduce the viscosity and make it easier to move to the storage tank.

On January 15, 1919, shortly after 12.40pm, a massive storage tank in Boston buckled and gave way, releasing a wave of molasses that measured nearly as high as a football goalpost, according to historical accounts.

Because the molasses was probably still several degrees warmer than the cold, winter air, when the tank broke, it chilled as it streamed through the streets.

The temperature made the molasses more viscous, and more difficult for people to disentangle themselves.

Outrunning it was out of the question: Sharp says the molasses raced through the cobblestone streets, propelled by the sheer weight of the goop.

It took only moments for the molasses to engulf the area around Commercial Street, and it reduced buildings to rubble and damaged an elevated train.

About half of the victims died after they were stuck, and one firefighter drowned in the molasses, according to The Boston Globe in 1968.

If the molasses leaked in the summer, the warmer temperatures would have thinned out the syrupy mess.

Just last year, structural and metallurgical engineer Ronald Mayville found the same steel that was used on the Titanic was used in the storage tank, which was 'stressed well beyond capacity' because it was too thin, according to the Boston Globe.

Nearly 100 years later, Harvard University researchers say the tsunami claimed so many lives because the cold, winter air chilled the molasses and caused it to become more viscous

Sharp's team combed through hundreds of pages of historical accounts. Researchers also studied century-old maps and archived National Weather Service meteorological data.

Harvard graduate student Jordan Kennedy analyzed the properties of blackstrap molasses and how it flows at different temperatures.

The team found that molasses thickens dramatically when exposed to cold, and that at the time of the collapse, the stuff in the storage tank was likely to be considerably warmer than the wintry air outside.

Two days before the disaster, the tank had been topped off with a fresh shipment of molasses from the balmy Caribbean that hadn't yet cooled to Boston winter temperatures.

Once the tank split and the molasses gushed across the Boston waterfront, it cooled rapidly, 'complicating attempts to rescue victims,' the team said in its report.

Mapping the physics of the molasses flood could help experts better understand other catastrophes such as industrial spills or ruptured levees, Sharp said.