A closer look
When the US military detonated a 15-megaton thermonuclear bomb over Bikini Atoll in 1954, they gave the ocean a permanent radioactive tramp stamp. The fallout from the Castle Bravo test scattered deadly radionuclides across the Pacific. Because coral skeletons grow in annual bands like trees, they absorbed the radioactive strontium-90 right into their calcium carbonate structures. Today, marine biologists use these glowing, microscopic scars as morbid geological clocks to track historic ocean temperatures and contamination.
