The United States tested its largest thermonuclear bomb in 1954 over Bikini Atoll in the Marshall Islands, generating radioactive fallout downwind, including over remote Rongelap Atoll.We surveyed protected reefs of Rongelap and neighboring Ailinginae Atoll, finding extremely variable coral condition and widespread evidence of recent ocean warming.Variation in reef condition underscored an increasing need to assist diver-based surveys with improved satellite and aircraft imaging to assess the health of the coral reefs.Climate change mitigation is paramount to coral reef survival, as increasing ocean temperature could trump earlier nuclear radiation as a driver of reef degradation in the Marshall Islands. On the morning of March 1, 1954, the United States tested its largest thermonuclear bomb over Bikini Atoll in the Marshall Islands. Code-named “Castle Bravo”, the explosion was more than 1,000 times more powerful than the Hiroshima and Nagasaki bombs that had ended World War II a decade earlier. Explosion of the Castle Bravo atomic bomb at Bikini Atoll in 1954. Photo: United States Department of Energy. The Castle Bravo test surprised its engineers, yielding twice as much power as had been predicted, and a massive fireball that destroyed three neighboring islands and contaminated 18,000 square kilometers (7,000 square miles) of ocean with radioactive fallout to the east of Bikini. Directly in the path of the Castle Bravo fallout was Rongelap Atoll, where radioactive debris up to 2.5 centimeters (1 inch) deep was deposited on the atoll’s small islands and extensive coral reefs. The tragic impact of the nuclear fallout on the people of Rongelap is well documented, involving hundreds of deaths from radiation poisoning and cancer, and lingering health effects that plagued several generations of survivors. The Rongelapese were eventually evacuated south to Kwajalein Atoll, but the damage to their community and the surrounding environment was devastating and could not be undone. Visiting Rongelap Atoll is no quick day trip. Few ships run that far north in the Marshall Islands, and doing so requires open-ocean transits fraught with potentially rough seas. We entered the country via the U.S. Army’s base on Kwajalein Atoll, then boarded a trawler converted for remote diving operations for the long ride north. We arrived on the outer edge of Rongelap as the sunrise cast light on iridescent green vegetation and white sand beaches. We saw neither boats nor even a single aircraft contrail during the entire 10-day trip. The utter remoteness of the region had come into sharp focus for our dive team.