San Francisco's fog is famous, especially in the summer, when weather conditions combine to create the characteristic cooling blanket that sits over the Bay Area.

But one fact many may not know about San Francisco's fog is that in 1950, the US military conducted a test to see whether it could be used to help spread a biological weapon in a "simulated germ-warfare attack." This was just the start of many such tests around the country that would go on in secret for years.

The test was a success, as Rebecca Kreston explains over at Discover Magazine, and "one of the largest human experiments in history."