At the height of the cold war, many Americans were digging up their back yards to build radioactive fallout shelters.
If the exploding bomb didn't get you, so the reasoning went, the radioactive fallout would.
What is radioactive fallout anyway? If you've seen a regular bomb exploding in a movie, you might have an idea of what fallout is. There's a loud boom, things fly up into the air, then they fall back down again.
Radioactive fallout is essentially this, fine radioactive dust that falls back to Earth. The dust that makes up fallout is extremely fine, and it works its way high into the atmosphere. It's so fine and high that it doesn't all fall hours, days, or even weeks after a nuclear blast.
In fact it takes hundreds of years for all the fallout from a single bomb to make it back to the surface.
What does this mean? There have been more than two thousand nuclear test explosions in human history, including over five hundred above ground tests. Today, only about fifteen percent of the fallout from those explosions has made it back to Earth. The rest is still up there.
It's hard to measure the continuing health consequences of radioactive fallout because you can't differentiate fallout cancer from cancer caused by anything else. Estimates on the number of contemporary fallout deaths from our decades of nuclear testing range wildly. One thing's certain though. The sky will keep falling for a long time to come.