The word bikini evokes beaches and a cartoon sponge, but it belongs first to Pikinni, meaning surface of coconuts, a Marshall Islands atoll whose 167 residents were asked in February 1946 to leave for the good of mankind. Between 1946 and 1958 the United States detonated 23 nuclear devices there, including the 15-megaton Castle Bravo shot that vaporized three islands and left a mile-wide crater.

This episode follows the Bikinians from their matrilineal land culture and asymmetrical proa canoes through exile on Rongerik, Kwajalein, and Kili, a 1972 return poisoned by cesium-137 in coconuts, and the recent depletion of a $59 million trust fund. It closes with the atoll’s strange present: thriving reef sharks, a sunken ghost fleet including USS Saratoga, restricted dive tourism, UNESCO status, SpongeBob mutation theories, and a swimsuit named in Paris four days after the first test.

• How the old wood effect skewed early radiocarbon dates and why permanent settlement is now placed around the first century CE
• Why the Navy needed a remote lagoon to test atomic sea warfare, and how the underwater Baker shot’s base surge coated ships and islands in fallout
• The lithium-7 miscalculation that made Castle Bravo roughly 1,000 times more powerful than the Hiroshima bomb
• Why potassium-starved coral soil led coconut trees to absorb cesium-137, producing an 11-fold rise in islanders’ body levels
• How the removal of audit limits in 2017 preceded spending on aircraft, real estate, and cargo ships that drained the fund to about $100,000