Oceanographers Map Legacy of Nuclear Tests at Bikini Atoll

Sonar scans reveal undersea craters from atomic testing conducted between 1946 and 1958

Bombed Seafloor
CSHEL University Of Delaware

In the 1940s and 1950s, the United States Army tested atomic bombs the seafloor in the central Pacific ocean. Now, for the first time, oceanographers have scanned the site near the Marshall Islands’ Bikini Atoll and mapped the simulated battlefield. The results were recently presented at the American Geophysical Union’s annual meeting.

Last June, marine scientist Art Trembanis of the University of Delaware led a team to the remote atoll in the Marshall Islands. The team used sonar to sweep the 180-foot-deep lagoon used as a test site, creating a map of the craters left behind by the detonations as well as the dozens of shipwrecks and other debris that litter the seafloor.

The National Park Service first surveyed the area in 1989 and 1990, before the era of sophisticated sonar, GPS and other mapping technologies. “While our maps were good, and based on many long hours in the water, they were not enough,” archeologist James Delgado, who led that initial mapping effort, says in the release. “We needed a more accurate map that could only be done when sonar and survey technology caught up with our needs as scientists.”

While mapping using sonar is now commonplace, the expedition faced difficulties. First, it takes six days to reach Bikini Atoll, including a 60-hour boat ride while keeping a steady eye on their exposure to harmful radiation.

Their first task was to map the site of Operation Crossroads, a pair of tests conducted in July 1946 to study how nuclear blasts affect ships. The U.S. anchored about 80 unmanned warships—some of which were German and Japanese—in the path of two blasts. (In total, 240 ships were used to support the operation, either as targets or to observe the detonations.) In the first test, dubbed Able, a bomb was dropped from a B-29 bomber jet and detonated above the water. In the second test, named Baker, the bomb was detonated 90 feet below the surface.

The team was not able to find any signs of the aerial blast, but evidence of the Baker test remains. The crater left by the blast was still visible and had not filled in with sediment as they predicted, reports Mindy Weisberger at Live Science. It is 26.2 feet deep with a diameter around 2,297 feet. Distinct ripples from the blast still radiate out from its center.

“[It’s] like if somebody dropped a very large pebble onto the sea bed,” Trembanis says. “It seemed as if Captain Marvel herself had punched the planet and put a dent in it.”

They also located the Japanese cruiser Sakawa, sunk during the test, which was missed by the previous survey.

Able and Baker, however, were minor compared the hydrogen bomb tests conducted at the site later in the 1950s. An oblong crater found at a depth of 184 feet was likely the remains of two overlapping blasts, they team concluded. The crater was formed by the 1954 “Castle Bravo” test—featuring a 15 megaton warhead, the largest nuclear device ever detonated by the U.S.—and “Castle Romeo” test, featuring an 11 megaton warhead.

In total, Weisberger reports the U.S. detonated 22 bombs at the Bikini Atoll between 1946 and 1958.

While the mapping project will provide researchers with new data on the blasts and their aftermath, Trembanis says in the press release that exploring such a complex underwater site is fascinating for oceanographers as well. He says the team would like to return and analyze more of the targets and shipwrecks their sonar flagged.

“In many ways, I was struck by it being this idyllic, beautiful Pacific island and I thought ‘This was the site of the most violent explosions on the planet.'" he says in the release. "It’s still very much a puzzle, and we’d love to be able to go back.”

The legacy of those blasts are still being felt today. Nick Perry at the Associated Press reports that the residents of Bikini were evacuated from the atoll and told their removal was only temporary. To this day, they have not been allowed to return to their home island due to nuclear contamination.

Get the latest stories in your inbox every weekday.