It was 2004, the day after Christmas, and thousands of European and American tourists had flocked to the beaches of Thailand, Sri Lanka and Indonesia to escape the winter chill in a tropical paradise.

At 7:59 AM, a 9.1-magnitude earthquake—one of the largest ever recorded—ripped through an undersea fault in the Indian Ocean, propelling a massive column of water toward unsuspecting shores. The Boxing Day tsunami would be the deadliest in recorded history, taking a staggering 230,000 lives in a matter of hours.

The city of Banda Aceh on the northern tip of Sumatra was closest to the powerful earthquake’s epicenter and the first waves arrived in just 20 minutes. It’s nearly impossible to imagine the 100-foot roiling mountain of water that engulfed the coastal city of 320,000, instantly killing more than 100,000 men, women and children. Buildings folded like houses of cards, trees and cars were swept up in the oil-black rapids and virtually no one caught in the deluge survived.

Thailand was next. With waves traveling 500 mph across the Indian Ocean, the tsunami hit the coastal provinces of Phang Nga and Phuket an hour and a half later. Despite the time-lapse, locals and tourists were caught completely unaware of the imminent destruction. Curious beachgoers even wandered out among the oddly receding waves, only to be chased down by a churning wall of water. The death toll in Thailand was nearly 5,400 including 2,000 foreign tourists.

An hour later, on the opposite side of the Indian Ocean, the waves struck the southeastern coast of India near the city of Chennai, pushing debris-choked water kilometers inland and killing more than 10,000 people, mostly women and children, since many of the men were out fishing. But some of the worst devastation was reserved for the island nation of Sri Lanka, where more than 30,000 people were swept away by the waves and hundreds of thousands left homeless.

As proof of the record-breaking strength of the tsunami, the last victims of the Boxing Day disaster perished nearly eight hours later when swelling seas and rogue waves caught swimmers by surprise in South Africa, 5,000 miles from the quake’s epicenter.

Vasily Titov is a tsunami researcher and forecaster with the National Oceanic and Atmospheric Administration Center for Tsunami Research. He credits the unsparing destructiveness of the 2004 Indian Ocean tsunami on the raw power of the earthquake that spawned it. The quake originated in a so-called megathrust fault, where heavy oceanic plates subduct beneath lighter continental plates.

“They are the largest faults in the world and they’re all underwater,” says Titov.

The 2004 quake ruptured a 900-mile stretch along the Indian and Australian plates 31 miles below the ocean floor. Rather than delivering one violent jolt, the quake lasted an unrelenting 10 minutes, releasing as much pent-up power as several thousand atomic bombs.

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In the process, massive segments of the ocean floor were forced upward an estimated 30 or 40 meters (up to 130 feet). The effect was like dropping the world’s largest pebble in the Indian Ocean with ripples the size of mountains extending out in all directions.

Titov emphasizes that tsunamis look nothing like the giant surfing break-style waves that many of us imagine.

“It’s a wave, but from the observer’s standpoint, you wouldn’t recognize it as a wave,” Titov says. “It’s more like the ocean turns into a white water river and floods everything in its path.”

Once caught in the raging waters, if the currents don’t pull you under, the debris will finish the job.

“In earthquakes, a certain number of people die but many more are injured. It’s completely reversed with tsunamis,” says Titov. “Almost no injuries, because it’s such a difficult disaster to survive.”

An earthquake and tsunami of the magnitude that struck in 2004 is so rare that catastrophic tsunamis are all but unknown in the long cultural histories of India and Sri Lanka, explains Jose Borrero, a tsunami researcher with the University of Southern California and director of eCoast, a marine consultancy based in New Zealand.

“[The Indian Ocean tsunami] came ashore in these places that had no natural warning either, because they were far enough away that they didn’t feel any of the earthquake,” says Borrero. “So without a natural warning, without an official warning and with no history of tsunamis, hitting coastlines full of people, that’s the perfect combination to cause a lot of death and destruction.”

Both Borrero and Titov took part in U.S. Geological Survey expeditions in early 2005 to measure the full extent of the tsunami that struck Sumatra. It was during these expeditions that scientists confirmed maximum wave heights of more than 131 feet on the northwestern tip of the island. Borrero remembers coming upon a colossal freighter loaded with bags of cement that had been flipped on its back with its propeller in the air.

“This was the most extreme tsunami event since 1960,” says Borrero, referring to the 8.6-magnitude Chilean earthquake and tsunami that punished the Pacific, including the leveling of Hilo, Hawaii, 15 hours after the quake.

Titov will never forget the scene of widespread devastation he witnessed on Sumatra even months after the tsunami waters had subsided.

“We took a boat all the way from the middle of the island up to Banda Aceh, the hardest hit area, and for hundreds of kilometers it was as if somebody had taken an eraser and erased everything underneath the 20-meter line,” says Titov. “The sheer scale of the destruction was just mind-boggling.”