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,
Imos 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. 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.
Imos prow pushed into the No. 1 hold of
Mont Blanc, on her starboard side. The blast was felt as far away as
Cape Breton () and
Prince Edward Island (). The Richmond Railway Yards and station were destroyed, killing 55 railway workers and destroying and damaging over 500 railway cars. The
North Street Station, one of the busiest in Canada, was badly damaged. The death toll could have been worse had it not been for the self-sacrifice of an Intercolonial Railway dispatcher,
Patrick Vincent (Vince) Coleman, operating at the railyard about from Pier 6, where the explosion occurred. He and his co-worker, William Lovett, learned of the dangerous cargo aboard the burning
Mont-Blanc from a sailor and began to flee. Coleman remembered that an incoming passenger train from
Saint John, New Brunswick, was due to arrive at the railyard within minutes. He returned to his post alone and continued to send out urgent telegraph messages to stop the train. Several variations of the message have been reported, among them this from the
Maritime Museum of the Atlantic: "Hold up the train. Ammunition ship afire in harbor making for Pier 6 and will explode. Guess this will be my last message. Good-bye boys." Coleman's message was responsible for bringing all incoming trains around Halifax to a halt. It was heard by other stations all along the Intercolonial Railway, helping railway officials to respond immediately. Whether Coleman's warning was instrumental in halting the overnight train from Saint John a safe distance from the blast at
Rockingham, saving the lives of about 300 railway passengers, is uncertain. However, it certainly sped the dispatch of relief to the blast site. Coleman was killed at his post. ==Rescue efforts==