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Enewetak Atoll

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Southeast USA

Evidence of tilting of the U.S. coast

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South Africa

Accurate sea level markers

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Roe Plains, Australia

The team’s first expedition

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Researchers take advantage of a celestial collision

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New Zealand

Uplift exposes a repository of Pliocene sediments

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Researchers study crater of hydrogen bomb test

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A satellite view of Enewetak, where samples drilled in the crater blasted by a hydrogen bomb test provide Pliocene sea level records. Photo: NASA

For ten years, between 1948 and 1958 the U.S. tested 43 atomic and hydrogen bombs at the Enewetak Atoll in the Pacific Ocean. Atolls are circular chains of islands created when a volcano subsides, leaving only the rim showing above the sea. Coral, which grows on top of the volcanic foundation, accurately marks sea level. As the foundation sinks, new coral grows, leaving a record of sea level change.

In 1985 an interdisciplinary team led by the USGS (U.S. Geological Survey) drilled into the floor of two craters left by hydrogen bomb tests on Enewetak and took core samples.  Among the samples, the researchers found corals, believed to have lived at sea level, and dating to the Pliocene. The team adjusted the present depth of the corals for subsidence that had occurred since the Pliocene. Based on the Enewetak data, the scientists came up with an estimate of Pliocene sea level of 20-25 meters (66–82 feet) above that of today.