During World War II, air-planes of the United States and its allies dropped very heavy bombs on enemy territory. The biggest bombs were called "blockbusters" because they would destroy an entire city block. They were filled with TNT, a powerful explosive, and they weighed a ton, 2,000 pounds, each. Then on August 6, 1945, an American air-plane dropped a single bomb on the city of Hiroshima, in Japan. Unlike t n t bombs, this one was made to explode in the air, about 1,500 feet above the ground.

When this bomb exploded, the flash could be seen for two hundred miles. The explosion flattened buildings in nearly four square miles-the same as hundreds of city blocks.
About 70,000 persons were killed. This was the first atomic bomb. All the tremendous heat and force of the explosion was caused by the splitting of tiny atoms, each so small that it takes billions of them to make a grain of sand or the point of a pin. The United States called upon the Japanese to surrender. No reply was received, so on August 9 a second atomic bomb, even more destructive than the first, was dropped on the Japanese city of Nagasaki. The next day the Japanese asked for peace.

The atomic bomb was the deadliest weapon of warfare history had known or even dreamed of. It was also the most expensive, for the United States spent two billion dollars to make it. And the existence of the bomb was one of the best-kept secrets of the war, though thousands of persons worked on it. Deadly though these atomic bombs were, within a few years after the end of World War II the United States had made many more atomic bombs that were far more powerful than the first ones; and the atomic bomb had also opened the way to an even deadlier bomb, the hydrogen bomb.