Castle BravoThe United States’ thermonuclear testing program Operation Castle commences with the first detonation, code named Castle Bravo, on a reef near Bikini Atoll. The most powerful hydrogen bomb detonation carried out by the American military during the 20th century, Castle Bravo has a yield of 15 megatons, three times greater than predicted, with a blast radius of over four miles. Radioactive fallout spreads over 7,000 square miles of the Pacific Ocean, including neighboring islands and a Japanese fishing boat, with both populations showing symptoms of radiation sickness shortly afterward, forcing evacuations and later reparations.