On the morning of December 6, 1917, the world’s fourth-largest man-made non-nuclear explosion obliterated the city of Halifax, Nova Scotia, in Canada. It is estimated that the blast, the result of an accident, released energy the equivalent of 2.9 kilotons of TNT. (“Little Boy,” the bomb dropped over Hiroshima, released about 19 kilotons.) In the photo above, the explosion reaches more than two miles up. It is the photo taken closest to the moment after the explosion.

The explosion, combined with a tsunami it created in Halifax Harbor, was the most devastating man-made blast until the nuclear age. Almost 2000 people were killed and many thousands more were injured.
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The photo at top was taken in London, England, in December 1952. There is nothing remarkable about the photo or the location or the month or the year in which it was taken. The photo was taken at two in the afternoon that December day.

The Great Smog of London, which killed more than 4000 people in one week, began on this date in 1952. The photo depicts it clearly. The combination of London’s typical fog with unusually high levels of coal smoke and vehicle exhaust produced the deadly condition, which also gave the world the word “smog” itself, which combines “smoke” and “fog.”

The modern-day environmental movement was kicked into action by this human-influenced weather event, and air pollution reduction laws started to be passed in London that very month.
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