What caused the Halifax Explosion of 1917?

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The Halifax Explosion was caused by a collision between two steamships in the harbour of Halifax, Nova Scotia, on the morning of December 6, 1917.1

The Norwegian ship Imo and the French vessel Mont-Blanc were on a collision course while navigating the harbour. The Mont-Blanc was secretly carrying a massive cargo of explosives intended for the World War I effort, including trinitrotoluene (TNT), picric acid, and benzol.1

Following the collision, drums of highly combustible benzol tipped over and ignited, causing a fire on the deck of the Mont-Blanc. The burning ship drifted toward a pier, and shortly after 9:04 am, the cargo detonated. The resulting blast and tsunami caused catastrophic destruction to the city, leading to thousands of casualties.1