Explosion: The Largest Explosion of WW1

The following article is published with the kind permission of the author Terry Cox, Hertfordshire Constabulary Great War Society and Dr. F.R.J. Newman PhD - editor of Trench Foot Notes. 


On 6 December 1917 a French ship carrying high explosives was involved in a low speed collision in Halifax Harbour Nova Scotia resulting in the largest ever
non-nuclear explosion causing 11,000 casualties

In 1906 the Canadian government took over Halifax Dockyard 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 and costly effort to develop the harbour and its waterfront facilities. The outbreak of the Great 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 under the supervision of Captain Superintendent Edward Harrington Martin; by 1917 there was a growing naval fleet in Halifax, including patrol ships, tugboats, and minesweepers.

The population of Halifax on one side of the harbour and the town of Dartmouth on the opposite side had increased to between 60,000 and 65,000 people by 1917. Convoys carried men, animals and supplies to the European theatre of war. Hospital ships also ferried wounded to the city and a new military hospital was constructed there.

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 would gather in Bedford Basin on the north-western side of Halifax harbour which was protected by two sets of anti-submarine nets and guarded by patrol ships of the Royal Canadian Navy.

Convoys departed under the protection of British cruisers and destroyers. A large army garrison protected the city with forts and batteries of guns. These factors drove a major military, industrial and residential expansion of the city and the weight of goods passing through Halifax harbour increased nearly nine fold. All neutral ships, bound for ports in North America were required to report to Halifax for inspection.

The Norwegian SS Imo had arrived from the Netherlands en route to New York to take on relief supplies for Belgium. The vessel arrived in Halifax on 3 December 1917 and following inspection as a neutral ship the Imo waited for two days to take on coal. It had intended to set sail on 5 December but her coaling was not completed that day until after the anti-submarine nets had been raised for the night.

The French freighter SS Mont-Blanc arrived at Halifax late on the afternoon of 5 December, The vessel was fully loaded with the explosives TNT and picric acid the highly flammable fuel benzole and to cap it all off a load of gun cotton.

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. She had to wait outside the harbour for morning when the nets would be lowered.

Navigating into or out of Bedford Basin required passage through a strait called “The Narrows”. Ships exiting were expected to keep to the starboard (right) side of the channel as they passed incoming traffic - in other words, vessels were required to pass port to port. Ships were restricted to a speed of 5 knots (a mere 6 mph) within the harbour.

The Imo with Pilot William Hayes on board was given permission to leave the harbour at 0730 on 6 December and set off at excess speed hoping to make up the time she had lost by being held within the anti-submarine nets for the previous night.

Francis Mackey, an experienced harbour pilot, had boarded Mont-Blanc on the evening of 5 December 1917. 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 was lowered. Mackey first spotted Imo when she was about 0.75 miles away and became concerned as her path appeared to be heading towards his ship's starboard side, as if to cut across his own course.

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. 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 in negation.

A collision was now inevitable. Imo's prow pushed into the No. 1 hold of Mont Blanc on her starboard side. The damage to Mont Blanc was not severe, but it toppled barrels that broke open and flooded the deck with benzol that quickly flowed into the hold. As Imo's engines kicked in to reverse she quickly 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 as the benzol spewed out from crushed drums on Mont-Blanc's decks. The fire rapidly became uncontrollable.

Blast cloud of the explosion

At 9:04am the out-of-control fire on board Mont-Blanc set off her highly explosive cargo. The ship was completely blown apart and a powerful blast wave radiated away from the explosion at more than 1,000 yards per second. Temperatures of 5,000 °C 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. Mont-Blanc's forward 90 mm gun, its barrel melted away, landed 3.5 miles north of the explosion site and the shank of her anchor, weighing half a ton, landed 2 miles south.

The scene of the explosion looking like something from Passchendaele

A cloud of white smoke rose to over 12,000 feet. The shock wave from the blast travelled through the earth at nearly 23 times the speed of sound and was felt over 130 miles away. An area of over 400 acres was completely destroyed by the explosion and the harbour floor was momentarily exposed by the volume of water that was displaced. A 60 foot high tsunami was formed by water surging in to fill the void.

Over 1,600 people were killed instantly and 9,000 were injured, more than 300 of whom later died. Every building within a 1.6 mile radius, over 12,000 in total, was destroyed or badly damaged. Hundreds of people who had been watching the fire from their homes were blinded when the blast wave shattered the windows in front of them. Roughly 5,900 eye injuries were reported, and 41 people lost their sight permanently.

Stoves and lamps overturned by the force of the blast sparked fires throughout Halifax where entire city blocks were caught up in the inferno, trapping residents inside their houses. Fire-fighter Billy Wells, who was thrown away from the explosion and had his clothes torn from his body, described the devastation survivors faced: "The sight was awful, with people hanging out of windows dead. Some with their heads missing and some thrown onto the overhead telegraph wires." He was the only member of the eight-man crew of the fire engine Patricia to survive.

Fire fighters and their horse drawn fire engines were brought from as far as 200 miles away by rail to help fight the fires. At the time Canada had only one motorised fire engine. Police and the military were drafted in from miles around to help with the rescue work. Rescuers came from America by train.

The Halifax Explosion was the largest artificial explosion until Hiroshima – which was seven times larger.

Originally created by city officials on the day of the explosion, the Halifax Relief Commission administered a $30 million relief fund with responsibility for the medical care, compensation and rehabilitation of those injured or disabled by the Explosion. In 1976, the Halifax Relief Commission was finally shut down and its remaining $1.5 million and 68 surviving dependents transferred to the Canada Pension Commission

Terry Cox February 2018