Piccadilly Circus, London 1952Getty
The deadly fog first rolled in during December of 1952.
It enveloped London in a thick blanket of pollutants, reducing visibility to just three feet in some areas and causing transportation to shut down.
According to researchers, the sky during this time turned completely dark and thousands of people suffered from breathing problems.
The fog finally lifted on December 9, but at the time, it was thought that at least 4,000 people had died, along with thousands of animals, and more than 150,000 people were hospitalized.
Later studies, however, estimate that the death count may actually have exceeded 12,000.
While it"s long been known that coal burning was the cause, researchers have struggled to identify the exact chemical processes that made this event, also known as the Great Smog of 1952, so deadly.