Most Canadian troops left overseas from Halifax aboard enormous peacetime ocean liners converted to troopships such as (sister ship of "Titanic") and as well as many smaller liners. The city also served as the return point for wounded soldiers returning on hospital ships. A new generation of gun batteries, searchlights and an anti-submarine net defended the harbour, manned by a large garrison of soldiers. The United States Navy established a naval air station on August 19, 1918 to operate seaplanes. The base closed shortly after the First Armistice at Compiègne. Halifax's limited 19th century housing and transit facilities were heavily burdened. In November 1917, a subway system plan was presented to City Hall, but the city did not pursue the scheme. The war was seen as a blessing for the city's economy, but in 1917 a French munitions ship, , collided with a Norwegian ship, . The collision sparked a fire on the munitions ship which was filled with 2,300 tons of wet and dry picric acid (used for making lyddite for artillery shells), 200 tons of trinitrotoluene (TNT), 10 tons of gun cotton, with drums of Benzol (High Octane fuel) stacked on her deck. On December 6, 1917, at 9:04:35 AM the munitions ship exploded in what was the largest man-made explosion before the first testing of an atomic bomb, and is still one of the largest non-nuclear man-made explosions. Items from the exploding ship landed away. The Halifax Explosion decimated the city's north end, killing roughly 2,000 inhabitants, injuring 9,000, and leaving tens of thousands homeless and without shelter. The following day a blizzard hit the city, hindering recovery efforts. Immediate help rushed in from the rest of Nova Scotia, New Brunswick, Prince Edward Island, and Newfoundland. In the following week more relief from other parts of North America arrived and donations were sent from around the world. The most celebrated effort came from the Boston Red Cross and the Massachusetts Public Safety Committee; as an enduring thank-you, since 1971 the province of Nova Scotia has donated the annual Christmas tree lit at the Boston Common in Boston. The explosion and the rebuilding which followed had important impacts on the city: reshaping the layout of North End neighbourhoods; creating a progressive housing development known as the Hydrostone; and hastening the move of railways to the South End of the city. The city's economy slumped after the war, although reconstruction from the Halifax Explosion brought new housing and infrastructure as well as the establishment of the Halifax Shipyard.