On December 6, 1917, Halifax Harbor was a bustling hive of wartime activity. The SS Mont-Blanc, a French cargo vessel packed to the gunwales with over 2,500 tons of volatile explosives like picric acid, TNT, and liquid guncotton, was entering the narrow strait called the Narrows. At the exact same time, the SS Imo, a Norwegian vessel aiding Belgian relief efforts, was desperately trying to leave.
Delayed by coal deliveries, the Imo's captain was eager to make up for lost time and accelerated down the harbor along the wrong side of the channel. Under standard nautical rules of the road, ships were expected to pass port-to-port. However, after the Imo veered into the Mont-Blanc's lane to avoid other harbor traffic, a disastrous breakdown in protocol occurred.
The Mont-Blanc sounded its whistle once to indicate it had the right of way, but the Imo responded with two blasts, stubbornly signalling that it refused to yield. A confused, agonizingly slow-motion standoff ensued as both ships blew their whistles in disagreement. By the time both captains ordered full reverse, momentum made impact unavoidable.
At 8:45 AM, the Imo's bow scraped against the Mont-Blanc, cutting into its hull and igniting spilled barrels of benzol fuel on deck. The crew of the Mont-Blanc fled in lifeboats, unable to warn citizens on shore that their floating vessel was a giant live bomb. Twenty minutes later, the Mont-Blanc detonated with the force of roughly 2.
9 kilotons of TNT. The blast flattened two square kilometers of Halifax, generated a 60-foot tsunami, killed nearly 2,000 people, and instantly became history's deadliest traffic accident.