When a cold snap hit the London area in December of 1952, residents simply did what they've been doing for centuries: they put more coal on the fire.
That plan backfired in a terrifying way, however, when millions of coal fires poured tons of sulfur dioxide and other pollutants into London's air. The Big Smoke, a.k.a. the London Smog of 1952, is blamed for some 12,000 deaths.
England's reaction, however, was swift and decisive. Never again would a deadly smog bring city life to a standstill, and thanks to environmental regulations, the air in London became cleaner than at any time since the 1700s.