The night before the mission, Kenji Matsuzaki could not sleep. For more than a year, Matsuzaki and a team of engineers had been developing their little robot—a bread-loaf-sized, red and white machine equipped with five propellers, a transparent dome, front and rear video cameras, and an array of lights and sensors. Nicknamed Little Sunfish, it was engineered to operate ­underwater, in total darkness, amid intense radiation. And after three months of testing, training, and fine-tuning, it was deemed ready to fulfill its mission: to find and photograph the melted-down radioactive fuel that had gone missing inside the Fukushima Daiichi nuclear power plant.

More than six years had passed since an earthquake and tsunami hammered northeastern Japan and reduced the Fukushima facility to radioactive ruin. In all that time, no one had been able to locate the hundreds of tons of fuel inside the three reactors that had suffered core meltdowns. The uranium fuel had overheated, turned into lava, and burned through its steel container. That much was known. What happened after that was the big question. Did all the fuel flow out of the reactors, or was some still inside? Did it pile up in a heap, spread out in a puddle, spatter on the walls? Without knowing the answers to those questions, it was nearly impossible to devise a plan to get rid of it. And getting rid of it is imperative. Every day, as much as 165 tons of groundwater seeps into the reactors, becoming contaminated with radiation. And there’s always the possibility that another earthquake or some other disaster could rupture the reactors again, sending radiation spilling out into the air, sea, or both.

Human beings couldn’t go into the heart of Fukushima’s reactors to find the missing fuel, though—at least not without absorbing a lethal dose of radiation. The job would have to be done by robots. But no robot had ever carried out such a mission before. Many had already tried and failed. Debris tripped them up. Yard-thick concrete walls threatened to block their wireless signals. Radiation fouled up their microprocessors and camera components. And so it fell to Matsuzaki, a shy-eyed , 41-year-old senior scientist with Toshiba’s nuclear technology branch, to help build a machine that wouldn’t end up as another one of the robot corpses already littering the reactors.

Just getting the Sunfish and its support gear into position inside the enormous concrete building that housed one of the crippled reactors took two days. Four separate teams took turns setting up the control panel, cable drum, and other equipment the robot would need to function. Even in full protective bodysuits, each group of workers could spend only a few minutes inside the structure, working by the light of portable electric lamps amid a thicket of machinery, pipes, and catwalks. When one team absorbed its maximum permitted daily dose of radiation, it was replaced by another group. Matsuzaki himself made two forays inside to put the final touches on the Sunfish, sweating inside his face mask and bodysuit in the summer heat, his nerves jumping each time his portable monitor dinged to indicate he’d received another increment of his allowable radiation dose.

The plan was for the Sunfish to spend three days mapping the debris and searching for signs of the missing fuel. Matsuzaki would monitor its progress from a control room about 500 yards away. He would be joined by a half-dozen top officials from his employer, Toshiba, and Tokyo Electric Power Company (Tepco), the mammoth utility that owns the plant. His success—or failure—would be broadcast daily around the world.