The Halifax Explosion triggered the largest man-made explosion before the nuclear age.

On my walk at Mount Olivet Catholic Cemetery in Halifax, Nova Scotia, I visited the memorials remembering the victims and unidentified Catholic dead who perished when a collision of the Mont-Blanc and the Imo ships in Halifax harbour triggered the largest man-made explosion before the nuclear age.

On December 6, 1917, a French cargo ship called the Mont-Blanc had sailed from New York, fully loaded with wartime explosives, headed into Halifax harbour to join a convoy bound for Europe to help in the war effort. She was not required to fly any warning flags, as it was a wartime security measure.

That same day, a Norwegian relief ship chartered by the Belgian Relief Commission was empty and heading out of the harbour toward New York to pick up supplies. However, on that cold December day, she was running behind schedule and moving faster than harbour rules allowed.

A series of signaling misunderstandings and evasive maneuvers by both crews led Imo to strike Mont-Blanc on her starboard bow around 8:45 a.m. — a low-speed collision, but enough to spark a fire among the benzol drums on deck.

The burning ship drifted toward the Halifax pier over the next twenty minutes, drawing crowds of onlookers to the waterfront and windows to watch the spectacle. At 9:04 a.m., Mont-Blanc’s cargo detonated in one of the largest artificial explosions in history.

Fragments found from the Mont-Blanc. 

The blast levelled roughly 2 square kilometres of Halifax’s north end, flattening homes, factories, and churches almost instantly. Pressure waves shattered windows up to 80 kilometres away, and the explosion generated a tsunami-like wave in the harbour. Roughly 1,600 people were killed instantly, and another 9,000 were injured. Sadly, many of the injured were people who had rushed to windows to watch the burning ship and were blinded or maimed by flying glass. To add to the misery, a blizzard dumped heavy snow on the city that same night, hampering rescue efforts and burying survivors trapped in the rubble.

News reached Boston quickly, and Massachusetts organized one of the first large-scale international disaster relief efforts in history sending doctors, nurses, and supplies by train. Halifax still sends Boston an annual Christmas tree in thanks, a tradition that continues today.

The legal fallout was messy and, in the end, largely inconclusive against any individual. Eventually, the Privy Council ruled that both ships were equally at fault — a 50/50 split of responsibility between Mont-Blanc and Imo for the navigational errors that led to the collision.

One large marker has been dedicated to the 20 graves that hold the remains of the unidentified.

December 6, 1917 and the final tally. Approximately 2,000 died, approximately 9,000 injured including an estimated 6,000 who were left permanently disfigured or disabled, around 200 people who lost their eyesight entirely from flying glass and roughly 25,000; nearly half of Halifax’s population at the time, were left without adequate housing in the aftermath.

Vincent Coleman, the railway dispatcher who famously sent a warning message moments before the blast. Mt. Olivet is holding about 300 explosion graves—marked distinctively today with yellow stakes. In the photo, a Canadian flag was added to the yellow marker.