Everyone in Japan knows what happened on the morning of Aug. 6, 1945. At 8:15 a.m. a nuclear bomb was dropped on Hiroshima. The city had been spared conventional bombing by the United States so that the effects of a nuclear weapon on an undamaged city could be assessed. The device detonated about 600 meters above the city.
We know about the terrible effects of the blast, of fire and of radiation, and we know about the horrific cost on human life. At least 66,000 people were killed and around 69,000 more were injured in the explosion alone. The U.S. Strategic Bombing Survey mapped Hiroshima after the explosion and made calculations about the bomb’s yield and destructive capacity. You may have seen photographs of the city before and after the bomb, and pictures of the mushroom cloud rising over Hiroshima in the moments after the blast.
But marine ecologist Mario Wannier made a discovery a few years ago that raised a chilling question no one had thought to ask. What happened to Hiroshima?
Not what event took place on that day, of course we know that, but what happened, physically, to the buildings of the city? Where did Hiroshima go?
The bomb exploded with a force equivalent to around 16,000 tons of TNT. About 70 percent of the city’s buildings were completely destroyed, with almost every structure in a 1.6 kilometer circle under the bomb completely obliterated. Only about 50 buildings of particular strong construction, such as the Bank of Hiroshima, remained intact. Much of the rest of the city was swept up in the vast mushroom cloud.
Wannier had taken samples of sand from the Motoujina Peninsula, 6 kilometers south of the hypocenter of the Hiroshima bomb, and was examining them to assess the health of the ecosystem. But what he discovered were tiny glass globules, thousands of them. He took samples from other beaches on the peninsula and examined them using electron microscopy and X-ray analysis.
The glass globules were fallout debris. Wannier estimates that these particles make up around 2.5 percent of the sand of Hiroshima beaches. It is the city itself, torn up in the blast, melted in the inferno of nuclear heat, and scattered. It is now mixed with sand of the beaches. Wannier calls the granules Hiroshimaites, following “trinitites” the name given to fallout granules at the test site for the Trinity nuclear bombs in New Mexico.
“Initially I was looking for microorganisms in beach sands as a proxy to gauge the health of shallow marine environments off the beach,” he told me. Instead he discovered a lost city.
Wannier found granules of Hiroshima in all six beaches he sampled around Motoujina, estimating the mass of the fallout debris down to a depth of 10 centimeters to be about 36 metric tons for those beaches. He estimates that the total amount of the fallout debris comes to thousands of tons.
“We have found fallout debris 12 kilometers to the southeast of the A-bomb explosion hypocenter, around Great Torii Gate, Miyajima island, and also some 30 kilometers to the northeast,” he says. “We assume that the distribution of fallout debris roughly matches the area covered by the atomic cloud.”
After the bombing of Hiroshima, and Nagasaki a few days later, the emergency responses were naturally focused on rescue and saving lives. Wannier and his colleagues, writing in the journal Anthropocene in April, reflect on the odd fact that no one had thought about what had happened to the city itself.
“Somehow, in this situation of extreme emergency,” the authors write, “the question of the whereabouts of the vanished urban built structures was not addressed.”
Their publication is the first description of fallout debris from a nuclear explosion in an urban environment.
Despite the fact that a more powerful bomb was dropped on Nagasaki, the physical effects of the explosion were not as dramatic as Hiroshima. That’s because the blast at Hiroshima was kept in by the surrounding mountains. However, Wannier expects that if someone looks around Nagasaki they will find similar granules as he found around Hiroshima.
“We have no samples from Nagasaki,” he says. “Based on the Hiroshima analogue, it is highly likely that similar melt debris will be found.”
Rowan Hooper is managing editor of New Scientist magazine. He tweets at @rowhoop and his book, “Superhuman: Life at the Extremes of Our Capacity,” is out now, published by Simon & Schuster.