Just before a five-ton plutonium bomb detonated one-third of a mile above the city, unleashing a fireball exceeding 540,000 degrees and propelling a blast that pulverized buildings and carbonized flesh over three square miles, Wada, 18, was at a streetcar terminal, taking a lunch break after a driving shift. Taniguchi, 16, was riding his bicycle while delivering mail; an undersized boy, he barely reached the pedals.