Animated map of nuclear explosions, 1945-1998
pinktentacle.com
pinktentacle.com
Low efficiency means that the nuclear reaction doesn't end with the explosion. The remaining heavy nucleus decay at their normal rate and radiate.
The biggest one, Tsar Bomba was luckily very efficient.
"It has been estimated that detonating the original 100 Mt design would have released fallout amounting to about 25 percent of all fallout emitted since the invention of nuclear weapons." http://en.wikipedia.org/wiki/Tsar_Bomba
The development of ICBMs, missile submarines, and so on, actually made us a lot safer by making the "strike first and strike hard" approach much less effective. The situation was more volatile back when it took a month or so to put together the bombs for a nuclear attack. When you look at it from such a ruthless perspective, the Cold War made a weird kind of sense.
MAD (mutually assured destruction) is not a pretty thing.
Semipalatinsk hosts three of Kazakhstan's four nuclear reactors. They also have one of the world's biggest uranium deposits.
The USA conducted 331 atmospheric nuclear tests, at least 86 of them on US mainland territory.