Halifax in Canada where I lived for 6 years was the site of the
largest man-made explosion until the atomic bomb came along. December 6, 1917, during the first world war, a loaded ammunition ship collided with another ship and blew up - about 2,000 people died and up to 9,000 were injured in the explosion and the snowstorm that followed the next morning. 325 acres (about 2 sq. km) of Halifax were levelled by the force of the blast.
The explosion was so strong that windows were broken in Truro, almost 80km away, and the shockwave was felt in Sydney, Cape Breton, which is about 250km away. In Halifax itself you can go and see a window in the church downtown where the outline of a priest was essentially photographed into the glass by the flash of the explosion; there's a
monument to the victims about 4km away from the site of the explosion, where the over half ton ship's anchor shank landed. One of its cannons went almost 7km in the opposite direction. Oppenheimer studied the effects of the blast in calculating the explosive power of the weapons that were eventually dropped on Hiroshima and Nagasaki.
Even bureaucratic
letters sent after the event are quite something to read.
As thanks to the people of Boston, who immediately sent a train full of supplies and help once they heard of the disaster, Halifax sends them a huge Christmas tree every year.