On December 6, 1917, the French freighter SS Mont-Blanc, fully laden with high explosives like picric acid and TNT, entered the Narrows of Halifax Harbour. At the same time, the Norwegian vessel SS Imo was exiting the inner harbor, running late and navigating on the wrong side of the channel to make up time. As the two ships approached each other head-on, a series of misunderstood whistle signals created absolute chaos.
The Imo refused to yield its position, operating under an informal pilot agreement, while the Mont-Blanc signaled that it possessed the right-of-way. Desperate maneuvers in the final seconds resulted in the Imo's bow glancing the Mont-Blanc's hull, sparking a fire among volatile benzol barrels on deck. Unable to extinguish the flames, the Mont-Blanc drifted toward the shore and detonated 20 minutes later, wiping out entire neighborhoods in a blast equivalent to nearly three kilotons of TNT.