Every year, the city of Boston gets its official municipal Christmas tree from the Canadian coastal city of Halifax, Nova Scotia. Why? Because Boston helped when Halifax needed it the most.

On Dec. 6, 1917, the Mont-Blanc, a cargo ship out of France was heading out from a stop in Halifax en route to Bordeaux, France, to deliver a load of explosives to help in the World War I war effort. While traveling through the Narrows just outside the port, the ship ran into the Imo, a Norwegian ship headed to New York for a load of relief supplies. Despite traveling at a very low speed (about one knot, or 1.5 mph), the collision ignited a fire on the Mont-Blanc. This wouldn’t have been a problem if the Mont-Blanc was carrying other cargo, but it was carrying explosives. In short, 20 minutes after the fire started, it spread to the huge stores of munitions onboard. The Mont-Blanc exploded.
In that moment, it was one of the biggest explosions in human history to that point. A combination of the blast, along with projected, flaming debris destroyed the Richmond neighborhood of Halifax at a cost of $35 million (in 1917 dollars). Even worse, more than 9,000 people were injured, and 2,000 died.
As news of the horrible tragedy spread around the world, Massachusetts governor Samuel McCall was among the first to respond…and send help. A train full of Red Cross-trained nurses and medical supplies made its way to Halifax. It wasn’t an easy trip—the train had to negotiate a blizzard. But the relief team got to coastal Nova Scotia, where they set up hospitals, built shelters for survivors and for thousands rendered suddenly homeless, and helped look for survivors buried in the snow.
The relief effort went on for weeks, which ran up against the Christmas season. No matter—the relief effort brought Christmas cheer, too. The Massachusetts relief contingent gave away presents to orphaned children and to families left homeless or injured from the explosion. Meanwhile, back in Boston, support continued to rally. Hundreds of thousands of dollars were raised, to the tune of $750,000 from the state of Massachusetts.

Nova Scotia slowly got back on its feet. It paid thanks one year later with a simple, poignant gesture: It sent a giant Christmas tree to the city of Boston and its people. In 1971, fearing that those in Boston and Halifax who had lived through the tragedy were dying out, and the story would die with them, Halifax renewed the idea, and sent Boston another Christmas tree. And they still do, every year.