Perhaps the most awesome display of power in the 20th century occurred on Nov. 1, 1952, when "Mike," the first hydrogen bomb, was detonated in the Eniwetok Atoll in the Marshall Islands. As the radioactivity billowed upward in an ominous red cloud, instruments showed that the density of neutrons in the blast exceeded that in an exploding star, or supernova. The physicist Herbert F. York called it "a moment when the world suddenly chilled." The atomic bomb was thought of as being limited in power, he said, but with "Mike," physicists had learned "to build bombs whose power was boundless." The explosion, estimated at 10.4 megatons, left behind a crater 200 feet deep and more than a mile across.

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