1917 maritime disaster in Halifax, Nova Scotia, Canada 
On the morning of 6 December 1917, the French cargo ship collided with the Norwegian vessel in the harbour of Halifax, Nova Scotia, Canada. "Mont-Blanc", laden with high explosives, caught fire and exploded, devastating the Richmond district of Halifax. At least 1,782 people were killed, largely in Halifax and Dartmouth, by the blast, debris, fires, or collapsed buildings, and an estimated 9,000 others were injured. The blast was the largest human-made explosion at the time. It released the equivalent energy of roughly .
"Mont-Blanc" was under orders from the French government to carry her cargo from New York City via Halifax to Bordeaux, France. At roughly 8:45 am, she collided at low speed, approximately one knot (), with the unladen "Imo", chartered by the Commission for Relief in Belgium to pick up a cargo of relief supplies in New York. On "Mont-Blanc", the impact damaged benzol barrels stored on deck, causing them to leak vapours which were ignited by sparks from the collision, setting off a fire on board that quickly grew out of control. Approximately 20 minutes later at 9:04:35 am, "Mont-Blanc" exploded.
Nearly all structures within an 800-metre (half-mile) radius, including the community of Richmond, were obliterated. A pressure wave snapped trees, bent iron rails, demolished buildings, grounded vessels (including "Imo", which was washed ashore by the ensuing tsunami), and scattered fragments of "Mont-Blanc" for kilometres. Across the harbour, in Dartmouth, there was also widespread damage. A tsunami created by the blast wiped out a community of Mi'kmaq who had lived in the Tufts Cove area for generations.
Relief efforts began almost immediately, and hospitals quickly became full. Rescue trains began arriving the day of the explosion from across Nova Scotia and New Brunswick while other trains from central Canada and the Northeastern United States were impeded by blizzards. Construction of temporary shelters to house the many people left homeless began soon after the disaster. The initial judicial inquiry found "Mont-Blanc" to have been responsible for the disaster, but a later appeal determined that both vessels were to blame. The North End of Halifax has several memorials to the victims of the explosion.
Background.
Dartmouth lies on the east shore of Halifax Harbour, and Halifax is on the west shore. By 1917, "Halifax's inner harbour had become a principal assembly point for merchant convoys leaving for Britain and France." Halifax and Dartmouth had thrived during times of war; the harbour was one of the British Royal Navy's most important bases in North America, a centre for wartime trade, and a home to privateers who harried the British Empire's enemies during the American Revolution, the Napoleonic Wars, and the War of 1812.
The completion of the Intercolonial Railway and its Deep Water Terminal in 1880 allowed for increased steamship trade and led to accelerated development of the port area, but Halifax faced an economic downturn in the 1890s as local factories struggled to compete with businesses in central Canada. The British garrison left the city in late 1905 and early 1906. The Canadian government took over the Halifax Dockyard (now CFB Halifax) from the Royal Navy. This dockyard later became the command centre of the Royal Canadian Navy upon its founding in 1910.
Just before the First World War, the Canadian government began a determined, costly effort to develop the harbour and waterfront facilities. The outbreak of the war brought Halifax back to prominence. As the Royal Canadian Navy had virtually no seaworthy ships of its own, the Royal Navy assumed responsibility for maintaining Atlantic trade routes by re-adopting Halifax as its North American base of operations. In 1915, management of the harbour fell under the control of the Royal Canadian Navy; by 1917 there was a growing naval fleet in Halifax, including patrol ships, tugboats, and minesweepers.
The population of Halifax/Dartmouth had increased to between 60,000 and 65,000 people by 1917. Convoys carried men, animals, and supplies to the European theatre of war. The two main points of departure were in Nova Scotia at Sydney, on Cape Breton Island, and Halifax. Hospital ships brought the wounded to the city, so a new military hospital was constructed.
The success of German U-boat attacks on ships crossing the Atlantic Ocean led the Allies to institute a convoy system to reduce losses while transporting goods and soldiers to Europe. Merchant ships gathered at Bedford Basin on the northwestern end of the harbour, which was protected by two sets of anti-submarine nets and guarded by patrol ships of the Royal Canadian Navy.
The convoys departed under the protection of British cruisers and destroyers. A large army garrison protected the city with forts, gun batteries, and anti-submarine nets. These factors drove a major military, industrial, and residential expansion of the city, and the weight of goods passing through the harbour increased nearly ninefold. All neutral ships bound for ports in North America were required to report to Halifax for inspection.
Disaster.
The Norwegian ship had sailed from the Netherlands en route to New York to take on relief supplies for Belgium, under the command of Haakon From. The ship arrived in Halifax on 3 December for neutral inspection and spent two days in Bedford Basin awaiting refuelling supplies. Though she had been given clearance to leave the port on 5 December, "Imo"'s departure was delayed because her coal load did not arrive until late that afternoon. The loading of fuel was not completed until after the anti-submarine nets had been raised for the night. Therefore, the vessel could not depart until the next morning.
The French cargo ship arrived from New York late on 5 December, under the command of Aimé Le Medec. The vessel was fully loaded with the explosives TNT and picric acid, the highly flammable fuel benzol and guncotton. She intended to join a slow convoy gathering in Bedford Basin readying to depart for Europe but was too late to enter the harbour before the nets were raised. Ships carrying dangerous cargo were not allowed into the harbour before the war, but the risks posed by German submarines had resulted in a relaxation of regulations.
Navigating into or out of Bedford Basin required passage through a strait called the Narrows. Ships were expected to keep close to the side of the channel situated on their starboard ("right"), and pass oncoming vessels "port to port", that is to keep them on their "left" side. Ships were restricted to a speed of within the harbour.
Collision and fire.
"Imo" was granted clearance to leave Bedford Basin by signals from the guard ship HMCS "Acadia" at approximately 7:30 on the morning of 6December, with Pilot William Hayes on board. The ship entered the Narrows well above the harbour's speed limit in an attempt to make up for the delay experienced in loading her coal. "Imo" met American tramp steamer SS "Clara" being piloted up the wrong (western) side of the harbour. The pilots agreed to pass starboard-to-starboard. Soon afterwards, "Imo" was forced to head even further towards the Dartmouth shore after passing the tugboat "Stella Maris", which was travelling up the harbour to Bedford Basin near mid-channel. Horatio Brannen, the captain of "Stella Maris", saw "Imo" approaching at excessive speed and ordered his ship closer to the western shore to avoid an accident.
Francis Mackey, an experienced harbour pilot, had boarded "Mont-Blanc" on the evening of 5 December 1917; he had asked about "special protections" such as a guard ship, given "Mont-Blanc"'s cargo, but no protections were put in place. "Mont-Blanc" started moving at 7:30 am on 6 December and was the second ship to enter the harbour as the anti-submarine net between Georges Island and Pier 21 opened for the morning. "Mont-Blanc" headed towards Bedford Basin on the Dartmouth side of the harbour. Mackey kept his eye on the ferry traffic between Halifax and Dartmouth and other small boats in the area. He first spotted "Imo" when she was about away and became concerned as her path appeared to be heading towards his ship's starboard side, as if to cut him off. Mackey gave a short blast of his ship's signal whistle to indicate that he had the right of way but was met with two short blasts from "Imo", indicating that the approaching vessel would not yield its position. The captain ordered "Mont-Blanc" to halt her engines and angle slightly to starboard, closer to the Dartmouth side of the Narrows. He let out another single blast of his whistle, hoping the other vessel would likewise move to starboard but was again met with a double-blast.
Sailors on nearby ships heard the series of signals and, realizing that a collision was imminent, gathered to watch as "Imo" bore down on "Mont-Blanc". Both ships had cut their engines by this point, but their momentum carried them towards each other at slow speed. Unable to ground his ship for fear of a shock that would set off his explosive cargo, Mackey ordered "Mont-Blanc" to steer hard to port (starboard helm) and crossed the bow of "Imo" in a last-second bid to avoid a collision. The two ships were almost parallel to each other, when "Imo" suddenly sent out three signal blasts, indicating the ship was reversing its engines. The combination of the cargoless ship's height in the water and the transverse thrust of her right-hand propeller caused the ship's head to swing into "Mont-Blanc". "Imo"'s prow pushed into the No. 1 hold of "Mont Blanc", on her starboard side.
The collision occurred at 8:45 am. The damage to "Mont Blanc" was not severe, but barrels of deck cargo toppled and broke open. This flooded the deck with benzol that quickly flowed into the hold. As "Imo"'s engines kicked in, she disengaged, which created sparks inside "Mont-Blanc"'s hull. These ignited the vapours from the benzol. A fire started at the water line and travelled quickly up the side of the ship. Surrounded by thick black smoke, and fearing she would explode almost immediately, the captain ordered the crew to abandon ship. A growing number of Halifax citizens gathered on the street or stood at the windows of their homes or businesses to watch the spectacular fire. The frantic crew of "Mont-Blanc" shouted from their two lifeboats to some of the other vessels that their ship was about to explode, but they could not be heard above the noise and confusion. As the lifeboats made their way across the harbour to the Dartmouth shore, the abandoned ship continued to drift and beached herself at Pier 6 near the foot of Richmond street.
Towing two scows at the time of the collision, "Stella Maris" responded immediately to the fire, anchoring the barges and steaming back towards Pier 6 to spray the burning ship with their fire hose. The tug's captain, Horatio H. Brannen, and his crew realized that the fire was too intense for their single hose and backed off from the burning "Mont Blanc". They were approached by a whaler from HMS "Highflyer" and later a steam pinnace belonging to HMCS "Niobe". Captain Brannen and Albert Mattison of "Niobe" agreed to secure a line to the French ship's stern so as to pull it away from the pier to avoid setting it on fire. The hawser initially produced was deemed too small and orders for a hawser came down. It was at this point that the blast occurred.
Explosion.
At 9:04:35 am the out-of-control fire on board "Mont-Blanc" set off her cargo of high explosives. The ship was completely blown apart and a powerful blast wave radiated away from the explosion initially at more than per second. Temperatures of and pressures of thousands of atmospheres accompanied the moment of detonation at the centre of the explosion. White-hot shards of iron fell down upon Halifax and Dartmouth.
A cloud of white smoke rose to at least . The blast was felt as far away as Cape Breton () and Prince Edward Island (). An area of over was completely destroyed by the explosion, and the harbour floor was momentarily exposed by the volume of water that was displaced. A tsunami was formed by water surging in to fill the void; it rose as high as above the high-water mark on the Halifax side of the harbour. "Imo" was carried onto the shore at Dartmouth by the tsunami. The blast killed all but one on the whaler, everyone on the pinnace and 21 of the 26