According to Einstein's theory of relativity, even the smallest bit of matter is equivalent to a tremendous amount of energy. For instance, two pounds of matter, if converted completely into energy, would produce energy equivalent to the explosive force of 22 megatons of high explosives. In 1939, based on German experiments that successfully split the uranium atom, physicists were able to explain the process of nuclear fission. When a large, unstable atomic nucleus splits (fissions) the result is two or more smaller, more stable nuclei accompanied by the release of tremendous amounts of energy and lingering, deadly radioactivity. The first application of this powerful technology was in warfare. A top-secret research effort in the U.S. known as the Manhattan Project studied the fission process, and eventually produced the first atomic bombs, which were dropped on the Japanese cities of Hiroshima and Nagasaki in 1945. These relatively small bombs, the only atomic weapons ever used in a war, each produced an explosion equivalent to more than 20,000 tons of TNT. The nuclear weapons of today are capable of producing explosions thousands of times more powerful.