Hungarian-American physicist Leo Szilard conceived of the idea of the nuclear chain reaction while waiting for a traffic light in Bloomsbury, London.

Leo Szilard was a Hungarian-born American physicist, biologist and inventor who made numerous important discoveries in nuclear physics and the biological sciences. He conceived the nuclear chain reaction in 1933, and patented the idea in 1936. In late 1939 he wrote the Einstein–Szilard letter for Albert Einstein's signature that resulted in the Manhattan Project that built the atomic bomb, and then in 1945 wrote the Szilard petition asking president Harry S. Truman to demonstrate the bomb without dropping it on civilians. According to György Marx, he was one of the Hungarian scientists known as The Martians.