Makoto Nagai was sitting in his third-floor office at 2:46 p.m. March 11 when the earthquake alarm buzzed. An orange LCD screen flashed 100 and 4, telling him the number of seconds before a quake would hit.

The warning jumped to 6, said Nagai, 55, head of the emergency response team in Sendai, some 128 km west of the epicenter of what became the strongest quake in Japan’s recorded history.

“I stood up, and my coffee cup bounced sideways off my desk,” he said. “We were in an earthquake-resistant building, yet an internal wall collapsed. Then people started to scream.”

As the magnitude-9 earthquake erupted, Kazuma Yokota, a 39-year-old inspector from the Nuclear Industrial and Safety Agency, crawled under his desk in fear the ceiling would collapse. He watched as cabinet brackets ripped out of the wall in his office at the Fukushima No. 1 nuclear plant.

Under the ocean floor, two 58-km-thick slabs of the Earth’s crust had heaved in an 80-million-year-old grinding conflict between tectonic plates.

The quake wrenched part of the coastline almost 3.6 meters toward the United States. About 15 minutes later, a relieved Yokota headed to the west gate of the Tokyo Electric Power Co. plant. He joined thousands of workers in orange, blue and white boiler suits, streaming up a hill to evacuation points spread over the site.

Maintenance technician Kazuhiko Matsumoto, 43, was in the turbine building of the idle No. 6 reactor, wrapping up work on air ducts when the shock waves arrived. He clung to the wall as the room blacked out.

“People were pushing,” he said, “Someone was shouting, ‘get out, quick, get out.’ “

The Fukushima No. 1 plant had 6,415 people on site. As Tepco took roll calls, officials found just two employees were missing.

The crisis appeared contained. The six reactor buildings, reinforced concrete and steel boxes, had survived; the three operating reactors had shut down as designed.

“We thought we’d have to monitor the situation, but that was about all,” said Yokota, who headed a seven-member NISA team at the plant.

What Yokota didn’t know was that the quake knocked out a power transformer about 9.6 km away, severing the connection to the grid. It would be another hour before events conspired to make the name Fukushima synonymous with the biggest nuclear disaster since Chernobyl.

After leaving the plant, Matsumoto had to abandon his car because the roads were cracked and jammed with traffic. He walked close to 10 km to his in-laws’ to meet up with his wife and two sons, aged 9 and 3.

Yokota and two of his team headed for the town of Okuma, 5 km away, where NISA kept an emergency office. They found it wrecked, the first sign emergency procedures were unraveling. “We couldn’t contact anybody for one or two hours,” he said.

Back at the Fukushima power plant, its director, Masao Yoshida, set up a control center in an earthquake-proof bunker. Yoshida, 56, had taken the post in June, his fourth stint at the 40-year-old facility.

Linked by a hotline to Tepco headquarters in Tokyo, the three-story bunker had two filtration systems to keep out radiation. Yoshida knew the Fukushima plant “inside out” and was ready to take charge, said Yokota, who later joined him.

The plant chief’s priority was to restore power. He could get electricity from 13 backup diesel generators to run pumps for cooling reactors. Each generator is the size of a locomotive and could power 14,400 homes.

“Most generators were located in basement 1 of the turbine buildings,” said Yasuo Arai, who works at community relations for Tepco, pointing to a diagram in a brochure. The turbine buildings holding eight of them were about 140 meters from the shore.

As Yoshida assessed damage, in Minamisoma, Mayor Katsunobu Sakurai was witnessing a far deadlier threat. From the rooftop of the government office, one of the highest in the city of 70,000 people, he looked toward the sea and saw what looked like a wall of sand pummeling through rows of houses.

“In those first moments we couldn’t comprehend what we were seeing,” Sakurai, 55, said. The tsunami surged 2.4 km inland, swallowing everyone in its path. Almost 1,500 town residents were killed or are missing.

The seabed had buckled along a 297-km stretch of fault line.

The upheaval hurled about 173 cu. km of ocean at the coast, enough to flood all of Manhattan 1.5 km deep, according to estimates by Roger Bilham, a University of Colorado seismologist.

Akira Tamura, 35, a control room manager at the Fukushima plant’s No. 2 reactor, was at home in Minamisoma when the quake struck. Tamura jumped in his car, navigating around flooded streets to pick up his wife at the florist’s where she worked.

At Tamura’s workplace, the tsunami crashed over a 2.5-km breakwater of 60,000 concrete blocks as well as a 5.5-meter high wall. The plant, built on rock 10 meters above sea level, was pummeled by a wave as high as 15 meters, according to Tepco estimates.

Seawater flooded the basements of turbine rooms, disabling 12 of the 13 generators and destroying electrical switching units. Water shorted circuitry, depriving the reactors of power for cooling, and triggering the nuclear disaster that Tepco was forced to combat with fire hoses and makeshift pumps.

One turbine building was flooded to a depth of 1.5 meters, said Hikaru Kuroda, chief of Tepco’s nuclear facility management group.

At about 3:41 p.m., the plant lost alternating-current power at the three operational reactors. Tepco immediately informed the government it had experienced “station blackout” as required by nuclear emergency regulations.

The only defenses left were banks of “coping” batteries designed to last no more than half a day. Once deployed, a nuclear plant would be termed “12-hours coping.”

“What that means is the clock has started ticking on restoring power,” said Edward Morse, a University of California at Berkeley professor of nuclear engineering. Batteries buy several hours to “work miracles,” he said.

At 4:36 p.m., Tepco acknowledged it had lost control of the reactors. At 7:03 p.m., Prime Minister Naoto Kan declared a nuclear emergency, prompting an evacuation of residents around the facility.

Inside the No. 1 unit, water began dropping in the morning of March 12. At 8:36 a.m., the reading showed zero centimeters as the fuel came into contact with the air. Within four hours, 1.7 meters of rods were exposed.

That afternoon, about 24 hours after the quake, a hydrogen explosion inside the No. 1 reactor building sent radiation levels spiking. A second threat emerged from spent-fuel pools at the top of the reactors. The rods in these also need to be cooled and covered with water.

After two further blasts in the next 62 hours, it was a March 15 fire around the spent fuel pool in reactor 4 that told Morse that Tepco was venturing outside the nuclear playbook.

With the reactor shut for maintenance, more than 1,300 rods were stored in the building where an unexplained blast destroyed the roof.

Tepco now had three reactors at risk of meltdown and a fourth with a fire spewing radiation into the air. With no means to cool the reactors, the company drafted fire engines, Chinook helicopters, concrete-pouring trucks and crowd-control cannons to inject, spray and dump thousands of tons of water onto the spent-fuel pools and into the reactors.

Twice, radiation levels at the plant reached 1 sievert an hour. Exposure for four hours at those levels might lead to death within four months, the U.S. Environmental Protection Agency said.

On March 29, Tepco turned on lights in the control room of the No. 4 reactor.

The next day, the two missing workers, Kazuhiko Kokubo, 24, and Yoshiki Terashima, 21, were found dead in the basement of the adjoining turbine building.