At 9:05 a.m., in the harbor of Halifax in the Canadian province of Nova Scotia, the most devastating manmade explosion in the pre-atomic age occurs when the Mont Blanc, a French munitions ship, explodes 20 minutes after colliding with another vessel. More than 1,800 people died.

As World War I raged in Europe, the port city of Halifax bustled with ships carrying troops, relief supplies, and munitions across the Atlantic Ocean. On the morning of December 6, the Norwegian vessel Imo left its mooring in Halifax harbor for New York City. At the same time, the French freighter Mont Blanc, its cargo hold packed with highly explosive munitions—2,300 tons of picric acid, 200 tons of TNT, 35 tons of high-octane gasoline, and 10 tons of gun cotton—was forging through the harbor’s narrows to join a military convoy that would escort it across the Atlantic.

At approximately 8:45 a.m., the two ships collided, setting the picric acid ablaze. The Mont Blanc was propelled toward the shore by its collision with the Imo, and the crew rapidly abandoned the ship, attempting without success to alert the harbor of the peril of the burning ship. Spectators gathered along the waterfront to witness the spectacle of the blazing ship, and minutes later it brushed by a harbor pier, setting it ablaze. The Halifax Fire Department responded quickly and was positioning its engine next to the nearest hydrant when the Mont Blanc exploded at 9:05 a.m. in a blinding white flash.

The massive explosion killed more than 1,800 people, injured another 9,000—including blinding 200—and destroyed almost the entire north end of the city of Halifax, including more than 1,600 homes. The resulting shock wave shattered windows 50 miles away, and the sound of the explosion could be heard hundreds of miles away.