At 8:45 AM, 6 December 1917, at the height of WWI, the Belgian Relief vessel Imo, through human error and negligence, collided with the French munitions carrier Mont Blanc in the narrowest part of Halifax harbour. Sparks generated by the collision ignited benzol stored on Mont Blanc's deck; the burning liquid then seeped into the holds, where it lit 2766 t of picric acid, TNT and guncotton. At 9:06 the munitions ship blew a mile high in the world's greatest man-made explosion up to that time.

Over 2.5 km2 of Halifax's industrial north end was totally levelled, either by the blast, the subsequent tidal wave or the raging fire caused when structures collapsed inward on roaring stoves and furnaces. Homes, offices, churches, factories, vessels, the railway station and freight yards - all were obliterated. Farther from the epicentre, Citadel Hill deflected shock waves away from south and west Halifax, where shattered windows and doors were the predominant damage. Across the harbour, Dartmouth suffered devastation to a lesser degree, since its north section was sparsely developed. The blast shattered windows in Truro, 100 km away, and was heard in Prince Edward Island.

Out of a population of less than 50 000, over 1600 people died and 9000 were injured, including 200 blinded by flying glass. Sixteen hundred buildings were destroyed and 12 000 damaged; 6000 people were homeless and 20 000 lacked adequate shelter. Total damage amounted to $35 million. The misery was compounded by a forced evacuation of the city, necessitated by fire threatening the main dockyard magazine, and by appalling weather conditions in the days immediately following the explosion.

Relief assistance was immediate and extensive. Trains from throughout the Maritimes and from central Canada and New England brought medical aid, food, clothing, building materials and skilled labourers. The continuing assistance provided by the Massachusetts Relief Committee was particularly noteworthy. Money donated by government, industry and individuals worldwide eventually totalled some $30 million, and was administered 1918-76 by the Halifax Relief Commission.