Seventy-three years after serving as the site of the world’s first underwater nuclear test, the seafloor around the Bikini Atoll remains scarred by finely detailed craters and littered with derelict ships.
Today, an interdisciplinary team of scientists is using sonar to assess the complex submarine environment. The results provide a sobering assessment of humanity’s capacity to alter nature.
“We’re revealing for the first time the forest and the trees of that early dawn of the nuclear age,” said Arthur Trembanis, an oceanographer from the University of Delaware who presented his team’s results at AGU’s Fall Meeting. “Now we can see the configuration of the seabed [around Bikini] and the disposition of the many ships that were sunk.”
Unleashing the Power of the Atom
In the aftermath of World War II, the U.S. Navy chose the Bikini Atoll for a series of controlled nuclear explosions. Between 1946 and 1958, 23 confirmed tests were conducted throughout the area.
Trembanis and his team studied Able and Baker, a pair of tests conducted in July 1946 as part of Operation Crossroads. Both Able and Baker involved plutonium fission bombs with a yield of between 21 and 23 kilotons, but they were deployed differently.
The Able test bomb, nicknamed Gilda, was dropped by plane and detonated 150 meters above the lagoon and target fleet of ships positioned there. Pressure waves from the resulting fireball depressed the ocean’s surface and sank several ships.
In contrast, Baker was the world’s first underwater test of a nuclear weapon. The bomb was anchored 27 meters below the surface of the lagoon and target fleet. The Baker explosion, captured in a series of well-known images, sent nearly 2 million metric tons of water, sand, and pulverized coral skyward in a plume over 2 kilometers high.
Milky Mud and Cauliflower Features
Nuclear testing at Bikini ended in 1958. After so many years, Trembanis brought no expectations about what, if anything, his team might find.
They began by using sonar to “mow the lawn,” motoring a tin boat back and forth across an area 1.5 times the size of Central Park. Altogether, the map represents 20 million individual points of reflected sound, the most detailed geoacoustic map of the region to date.
When scans of the Able site yielded undisturbed seafloor, it seemed time had reclaimed the evidence. But images of the Baker site revealed something unexpected.
Clustering around a laptop, Trembanis and his team witnessed the real-time rendering of an underwater crater more than 800 meters across—big enough to fit three Roman Colosseums.
Rather than a smooth bowl, radiating out from the center of the Baker crater was a series of what Trembanis called “cauliflower features” embedded in a “powdery, milky mud,” testaments to the blast’s immense force. He believes these structures formed as superheated debris from the cauliflower-shaped plume atop the spray column rained back down into the crater.
“If you imagine a bathtub, it would be like dumping a giant bag of sand into it,” Trembanis said. “It’s going to hit and then radiate away.”
Ghosts of the Past
Littered throughout the atoll are the husks of strategically placed ships—decommissioned dreadnaughts, aircraft carriers, and submarines meant to bear the brunt of the Able and Baker explosions.
In addition to their seafloor mapping, which Trembanis describes as “painting the house with broad brushstrokes,” the team performed detailed assessments of the 12 shipwrecks nearest the blast sites.
Both explosions sank vessels, flash melting ships into twisted specters. The USS Pilotfish, a submarine close to the Baker blast, was built to withstand several hundred pounds per square inch. But pressure sensors deployed during the test registered pressures 10 times higher.
Following up on the sonar findings, divers were sent to six wrecks, and all showed substantial damage that could have come only from these immense explosions. The Pilotfish rested on the bottom, its steel rivets torn apart when its hulls breached.
Even independent of their place in nuclear history, the Pilotfish and other Bikini shipwrecks attest to the long-lived effects of human activities on the environment.
As old ships decompose, they become ecological burdens, and researchers found that several wrecks on the Bikini seafloor are leaching plumes of oil. Trembanis and his team looked at sketches from surveys the National Park Service conducted in the late 1980s and saw the degradation of the past 40 years.
Visible and Invisible Scars
“Mapping the seafloor or shipwrecks isn’t new,” said Nicole Raineault, chief scientist with the Ocean Exploration Trust who was not involved in the study. “But mapping the impacts of a historical maritime event and being the first to monitor recovery adds something different.”
According to Trembanis, this study will serve as an important baseline for monitoring recovery: “Even though we think of the testing as having ended and gone away, the impact on both the people and the environment is still quite visible.”
—Amanda Heidt (@Scatter_Cushion), Science Communication Program Graduate Student, University of California, Santa Cruz