Tsunamis -- a combination of the words for "harbor" and "wave" -- occur most often in the Pacific Ocean, threatening not only Japan but Alaska, Hawaii, Chile and Peru.
But the Indian Ocean -- the site of last weekend's massive tsunamis -- has experienced relatively few. The last such deadly wave struck India in 1945, killing several hundred people.
But with a current death toll expected to rise past 70,000, no one in South Asia is likely to think of tsunamis as a distant danger ever again.
One after another, survivors of the disaster have described the terror of seeing a placid sea suddenly recede toward the horizon before rising into a massive wall of water rushing onto land faster than they could run.
Reuters news agency interviewed Australian Bob Philips, who was vacationing in Sri Lanka when the tsunami hit.
"The water came up to my door. And the water was so powerful, my bungalow just exploded. I saw the roof go and the walls just caved in. The water carried me about a kilometer into the lagoon," Philips said.
Tsunamis can be spurred by volcanic eruptions and underwater landslides -- even, theoretically, by an asteroid crashing to Earth.
But some of the most powerful waves are, like those in South Asia, caused by earthquakes near or on the ocean floor.
The 26 December earthquake was the largest anywhere in the world in the last 40 years, with a magnitude of 9.0. It was caused by the sudden collision of two tectonic plates under the floor of the Indian Ocean.
The collision caused a stretch of seabed over 1000 kilometers long to suddenly shift, jolting the waters above, and sending out in all directions a series of waves from the epicenter, close to the Indonesian island of Sumatra.
The waves struck parts of the Sumatran coast within minutes. But to the north, west and south, they continued to ripple through the water at speeds approaching 800 kilometers an hour.
In deep sea, tsunamis are barely detectable, often raising the water's surface no more than a normal wave. A boat just kilometers from the coast can sail over tsunami ripples with little or no disturbance.
David Galloway, a seismologist with the British Geological Survey in Edinburgh, Scotland, describes what happens as the wave approaches shallower coastal waters.
"Basically, this is what happens: When the earthquake occurs, basically the sea slumps, rather like losing the water in your bath going down the plughole. And obviously that water then has to come back, and that's what creates all the waves. And these tsunami waves can be devastating. They're very fast. But when they start getting near the coastal lines, they slow up, but they also get larger, and that's when they're very devastating to people in coastlines," Galloway said.
One of the highest tsunamis on record occurred in a remote Alaskan bay in 1958, when an earthquake and subsequent landslide sparked a wave that reached an incredible 520 meters -- nearly twice the size of the Eiffel Tower.
The recent tsunamis were far smaller, rising to maximum heights of 10 meters. But their impact along South Asia's densely populated coastal regions was devastating.
The ocean first receded and then the waves crashed down, sucking thousands of people out to sea or slamming them against buildings, cars, and trees. Others drowned as the water rushed inland as far as 2 kilometers. Then the sea would retreat, only to be followed by another, sometimes more powerful wave.
The deadly effects of the tsunami were felt as far away as Africa, where the waves slammed against the Somali coast some six hours after the earthquake's first tremblor, killing hundreds of people.
The waves took between two and four hours to hit land in India, Sri Lanka, and the Maldives.
That travel time could have left ample opportunity to warn people to move away from the coast. But unlike the Pacific, which has had a tsunami warning system since 1965, the Indian Ocean is not monitored for wave activity.
Galloway says tsunamis are simple to detect, even hours before they hit land.
"They're very easily detected in the Pacific Ocean because they have warning centers there, so they can actually give people along coastal areas a warning to stay away. But unfortunately, in the Indian Ocean, there aren't so many warning centers set up. So they only knew about it when the actual waves hit. If there had been a warning center set up, they might have had extra time. Even if it was minutes, they could have given a warning to people to get off the beach and away from coastal areas," Galloway said.
"The New York Times" reports that scientists at the Pacific Tsunami Warning Center based in Hawaii registered the Sumatra earthquake, and within 15 minutes had sent warnings to 26 nations, including Thailand and Indonesia.
Phone calls also were made to other Indian Ocean countries. But with no regional emergency plan in place, it was impossible to spread the alert as quickly as necessary.
Officials have already called for an early-warning system to be established for the Indian Ocean. But for the 70,000 dead and millions more devastated by Sunday's tsunamis, it will come too late.