BOSTON (AP) — Harvard University researchers say they’ve solved the mystery behind one of Boston’s most peculiar disasters — the Great Molasses Flood of 1919.

Twenty-one people were killed, 150 others were injured and entire buildings were flattened when a giant storage tank ruptured on Jan. 15, 1919. It sent a sticky tsunami of molasses sweeping through a crowded North End neighborhood.

The Harvard scientists who studied the disaster say they’ve determined why the loss of life was so great. They say the tank had just been topped off with warm molasses from the balmy Caribbean, and it hadn’t yet cooled to Boston winter temperatures when the tank gave way.

Once the molasses made contact with the chilly air, it rapidly thickened, complicating attempts to rescue people trapped in the goop.

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