On the afternoon of March 11th—a Friday—the Fukushima Daiichi Nuclear Power Station, on Japan’s Pacific coast, had more than six thousand workers inside. The plant, about a hundred and fifty miles north of Tokyo, is painted white and pale blue and is a labyrinth of boxy buildings and piping on a campus larger than the Pentagon’s. It has six reactors. Yusuke Tataki was in the concrete building that contains Reactor No. 4. He is a tobi worker—a scaffolder—and, at thirty-three, is small and nimble. Like many of the plant’s employees, he grew up nearby. He often skipped classes to surf, and after high school he worked, unhappily, on an assembly line welding circuit boards, until he got into scaffolding, which has kept him employed, on and off, at nuclear plants ever since. “Even when there’s no work elsewhere, there is work at the plants,” he told me recently.

At 2:46 P.M. on March 11th, an earthquake began to rattle the building, more violent than any Tataki could remember. It knocked him to the ground, and the lights failed. In darkness, he heard steel crashing against steel and men shouting. The building groaned. Heavy objects fell around him from heights of three or four stories. When the shaking stopped and emergency lights came on, the air was thick with a chalky haze of dust and concrete. Tataki knew that the only way out of the building was through an air lock—a set of parallel doors designed to prevent contamination—but the quake had jammed it shut. The workers around him were disciplined but anxious, and they banged on the door for help. “We all knew that during a quake everything in there could become contaminated with radiation,” he told me.

The quake had erupted beneath the ocean floor two hundred and thirty miles northeast of Tokyo, at a magnitude of 9.0—the strongest ever recorded in Japan. On that day, three of the Fukushima plant’s reactors were down for routine maintenance; the other three shut down automatically, as intended.

After a few minutes, a guard managed to open the jammed door, and Tataki hurried toward the parking lot. He has a wife and two children, and wanted to check on them and on his house. About seven hundred employees remained. “We had trained for this,” Keiichi Kakuta, a forty-two-year-old father of two, who worked in the public-affairs department, told me. “Of course, the mood was tense, but after someone said the reactors had shut down I saw the plant chief calmly instructing people what to do.” To ride out the aftershocks, Kakuta holed up, with most of the others, in the plant’s radiation-resistant Emergency Crisis Headquarters, situated on a small slope nearby. Japanese authorities had picked up the trail of a tsunami sweeping across the Pacific, but its precise approach was still a mystery.

In the imagination, tsunamis are a single towering wave, but often they arrive in a crescendo, which is a cruel fact. After the first wave, survivors in Japan ventured down to the water’s edge to survey who could be saved, only to be swept away by the second. In all, twenty thousand people died or disappeared along a stretch of the Japanese coast greater than the distance from New York to Providence.

At the plant, the first wave arrived at 3:27 P.M., but it did not overtop a thirty-three-foot concrete seawall. Eight minutes later, the second wave appeared: a churning white mass of water, four stories tall, that leaped over sixty thousand concrete blocks and barriers—designed to defend against typhoons, not tsunamis—and advanced toward the reactors. First, the water approached the turbine buildings, which had been built with large shutters facing the sea. It burst through the closed shutters and swamped the buildings. Inside, the plant’s emergency diesel generators, each the size of an eighteen-wheeler, were stored on the ground floor and in the basements. They were destroyed, and two workers who had been sent underground to check for leaks were killed. The water hurled pickup trucks pinwheeling end over end into delicate pipes and equipment, and it swamped the campus in roiling brown pools, fifteen feet deep, leaving the nuclear reactors protruding like boulders in a river. And then it recoiled into the sea.

Two minutes after the water arrived, the plant’s main control rooms began to lose electrical power. Hundreds of gauges and instruments went dark or froze. A worker who was keeping a log of the deteriorating situation scribbled something unprecedented in a Japanese nuclear plant: “SBO”—station blackout. Kakuta said to himself, “What happens now?” Without a constant source of coolant, the nuclear fuel rods in the heart of the three active reactors would eventually boil away the water that prevents them from melting down. The log entry was grim: “Water levels unknown.” Workers desperate for electricity had to improvise: they fanned out into the parking lot to scavenge car batteries from any vehicles that had survived the wave.

When the earthquake struck, the top ranks of Japan’s government were already in one place: at a meeting in a wood-panelled chamber of the national legislature, in Tokyo, discussing whether the Prime Minister, Naoto Kan, had taken illegal campaign donations. Kan was the fifth Prime Minister in four years; Japanese politics had become so fractious and unstable in recent times that when the German newspaper Die Zeit produced an illustration of Kan it mistakenly featured a Prime Minister who was booted from office two years earlier. Kan, a populist whose temper had earned him the nickname Irritable Kan, was sixty-four; his cabinet included younger officials, who conveyed an air of reform. At 7:03 P.M., Kan declared a nuclear emergency, and shortly afterward he ordered people within three kilometres of the plant to evacuate; the number of nuclear refugees eventually expanded to eighty thousand. But that evening Kan’s spokesman, Yukio Edano, projected calm: “Let me repeat that there is no radiation leak, nor will there be a leak.”

Gregory Jaczko, the chairman of the United States Nuclear Regulatory Commission, learned of the tsunami before dawn, in Washington, D.C., from his clock radio. The quake had struck overnight, D.C. time, and his first concern was about any tsunami effects on the West Coast of the United States. Once it was clear that there were none, he turned to Fukushima, but his staff was having a difficult time determining the details of what was happening. Over the phone, Japanese counterparts were vague or inconsistent, leaving exasperated American officials at the N.R.C.’s headquarters, in Rockville, Maryland, to watch the unfolding drama helplessly. “We were getting a lot of our information from the same source that everybody else was: open source, CNN, Japanese television,” Patricia Milligan, a senior N.R.C. emergency-preparedness specialist, recalled. Initially, Jaczko was not surprised; the tsunami had created an epic humanitarian crisis. He figured Japanese authorities were too busy to keep him updated. But gradually he reached a more disturbing conclusion. “It wasn’t a question of them not providing the information to us,” he told me. “The information just didn’t exist.”

The story of how Japan became one of the world’s most devoted, and improbable, advocates of nuclear technology begins in August, 1945. In the days immediately after the United States dropped atomic bombs on Hiroshima and Nagasaki, radiation was largely a mystery to the Japanese public; men and women had survived the explosion but were succumbing to a new illness—an “evil spirit,” as a national newspaper put it.