From its very beginnings, the atomic bomb has been mysterious. Even the physicists who lived together in Los Alamos to develop it did not know what they had invented. They didn’t know how powerful the first Bomb would be—they had a betting pool on the yield, and many seriously underestimated and overestimated the result. About half the scientists didn't think the device would explode at all. Enrico Fermi was taking bets on whether it would burn off the Earth's atmosphere.

But most of the mystery was deliberate. The Bomb was developed in complete secrecy so as not to tip off the Nazis, who were believed to be working on their own Bomb project. Even after Germany’s defeat, the Bomb was kept secret from the remaining enemy of Japan, but also from America’s war allies. Then after the war, as the truth of what the Bomb’s effects became clear to scientists, the American military and Washington policymakers tried to keep some of those effects secret from U.S. citizens, even to the point of outright lies.

The Bomb produces three effects: blast, heat and radioactivity (commonly called radiation.) The blast is immensely more powerful, and the heat is immensely more intense, than any other manmade device can produce. Together they resulted, in the Bomb’s first test, in killing every living thing within a mile, including insects. A single Bomb each virtually leveled the cities of Hiroshima and Nagasaki. Human beings were vaporized. Nothing was left of some but their shadows burnt into concrete. Others were seared to a small pile of ashes. The remains of some were fused with metal doors and other objects.

Those effects were immediately apparent. But it took some time for the effects of radiation to be understood, and even longer to be acknowledged.