By Michael Brennan, Ph.D.
In July 1946, two first-generation atomic bombs rocked the remote North Pacific island of Bikini Atoll in the Marshall Islands. Part of Operation Crossroads, a massive-scale endeavor to study the effects of the new atomic weapon on a naval fleet, the two test detonations—an aerial drop from a B-29 and one suspended in the water column—codenamed Able and Baker, targeted an array of more than 90 legacy ships from World War II. In total, 12 of the target ships sank to the lagoon bottom, including the renowned aircraft carrier USS Saratoga. While there was opposition to the tests before they were conducted and the fallout from them afterward would prove substantial, including the cancellation of a third planned test in 1947 after an inability to decontaminate the target ships following the Baker test, from a research perspective the operation led to greater understanding of nuclear power and its effects both at the time and in subsequent follow-on studies.
In June 2019, a team of archaeologists from SEARCH Inc. and oceanographers from the University of Delaware made the journey back to Bikini Atoll to revisit the sunken fleet once again. Utilizing new sonar and mapping technologies, they created the first detailed, scientific map of the submerged physical record of Operation Crossroads and its simulated naval atomic battlefield.
STUDYING OVER TIME
Immense efforts were undertaken to document and gauge the effects of the two detonations as they occurred. Still, many of the effects of the blasts remained hidden beneath the lagoon. In 1947, a scientific resurvey team conducted over 600 dives with Navy divers to some of the wrecks, focused particularly on the Saratoga, the Japanese battleship IJN Nagato, and the submarines USS Apogon and USS Pilotfish. However, the divers had difficulty surveying the ships underwater because of poor visibility due to a thick layer of loose mud that buried much of the site and parts of the wrecks.
The shipwrecks were not studied again until 1989 and 1990, when a team of archaeologists from the National Park Service’s Submerged Cultural Resources Unit conducted dives to document and evaluate the ships with support from the Navy and the Department of Energy. The team conducted dives on nine of the wrecks, with an emphasis on the Saratoga. Each wreck was determined to be both historically and archaeologically significant due to their unique destruction and as components of the only simulated nuclear battlefield in history. Photographs and a detailed site plan of the Saratoga, as well as sketches of other wrecks and site descriptions for each ship, were produced by the team.
It was the first archaeological and scientific look at the sunken fleet in four decades—and the first time the naval legacy of the Atomic Age was studied and shared with the public. Unfortunately, time was limited, and the surveying technology that was available meant that much of the fleet and the seabed around it could not be examined.
REVISITING THE WRECKS
Despite the large amount of data collected during the atomic test in 1946, and the subsequent resurveys of 1947 and 1989-1990, further research was important. The wrecks are now test instrumentation of not only the effects of atomic blasts, but also, decades later, the means to gauge the effects of time in the tropical lagoon waters on World War II-era warships. The mission in 2019 brought new sonar imaging technology to document the ships and the damage caused by the blasts in greater detail, as well as characterizing how the wrecks look nearly 75 years later.
The team used a 3DSS-iDX-450 3D side-scan sonar by PingDSP to survey the entirety of the lagoon bottom, where the nuclear blast impacted the seabed. This look revealed a graveyard of warships slowly corroding in the tropical atoll waters and a pronounced crater, still preserved seven decades later, punched into the seabed. Dominating the submarine landscape, the Baker crater had never before been mapped in 3D. Scientists in 1947 could only estimate its size and depth. The team had expected the crater in the seabed from Baker to have filled in over time; instead, it has remained prominent, possibly even more so now, as the divers decades ago had noted that a layer of suspended sediment covered the seabed.
Also apparent is the wreck of the Japanese cruiser, IJN Sakawa, which was sunk by the Able blast. When Baker was detonated, nearly right over the spot where the ship had been sunk three weeks prior, it flattened the cruiser into the seabed. Likely obscured by the sediment layer, divers had been unable to find the Sakawa previously, though they suspected it lay in the crater. Wreck divers rediscovered it after the archaeological dives in 1990, but this was the first archaeological documentation.
In the October 1947 issue of The Military Engineer, an article by research chemist Jack de Ment entitled “Instruments of Operation Crossroads” outlined the engineering of the test instrumentation setup at Bikini Atoll. Engineers from the U.S. Navy and U.S. Army and scientists from Los Alamos National Laboratory had set up hundreds of sensors, monitoring stations, and cameras to document the blasts and their effects. The end result was, at the time, the most instrumented and documented experiment in history.
While expansive and complex, the major objective of the operation was to determine if and how a naval fleet could survive an atomic attack and keep fighting. The Able test’s Mk 3 plutonium core weapon (the same type dropped on Nagasaki) detonated 513-ft above the attack transport USS Gilliam, flash-melting the metal of the ship and sinking it in less than a minute. Windows on the battleship USS Nevada, positioned only a few yards from the blast, shattered rather than melting. De Ment postulated this was not because the heat was insufficient to melt the glass, but because the blast was so instantaneous it precluded fusion, which he compared to passing a combustible material so quickly through a flame it does not catch fire. Such results from Operation Crossroads highlight how early we were in our understanding of atomic weapons.
Other ships positioned close to the zero points of the two detonations reveal the catastrophic effects of the heat and pressure waves produced by atomic blasts.
The hull of the wreck of the USS Gilliam appears like melted candle wax drooping from being flash-melted by the momentary heat of the Able blast and cooling as it sank beneath the lagoon less than a minute later. The submarine USS Pilotfish was submerged for the underwater Baker test, its stern facing the detonation, and its wreck shows the effects of the 5,200-psi pressure wave that crumpled the metal skin of the submarine around the frames as if it had been shrink wrapped. The air inside the submarine had nowhere to go and was forced out the bow through a hole blown in the pressure hull near the forward torpedo room.
The wreck of the Saratoga, well-studied and documented by both the Navy divers in 1947 and the National Park Service team in 1989 and 1990, still has more to say as well. The starboard side, which had been facing the Baker detonation, shows buckling along the hull from the impact of the pressure wave, which generated tsunamis that pushed the carrier a few hundred meters from where it had been moored for the test. The aft section of the carrier is also indicative of the ship’s time underwater. The sonar mapping documented a large area near the stern that has begun collapsing when compared to the National Park Service drawings, likely from accelerated corrosion from the tropical waters and microfractures in the steel caused by the impact of the blast.
CONTINUING THE LEGACY
Moving forward, there is more work that can be done in documenting the legacy of Operation Crossroads. With advancements in sonar and imaging, further research can be carried out on this remote resting spot in the Pacific Ocean.
The shipwrecks on the lagoon bottom of Bikini Atoll are more than a collection of naval vessels. They represent a cultural landscape that preserves the material evidence of the extensive effort to begin to understand the military implications of nuclear weapons at the dawn of the atomic age.
Michael Brennan, Ph.D., is Maritime Archaeologist, SEARCH Inc.; firstname.lastname@example.org.
[This article first published in the July-Aug 2020 issue of The Military Engineer.]