Tsunami. The word sounds so beautiful. Foreign, exotic, you might think it was the latest Far East must-have delicacy or the next Big Thing in Japanese cartoons. But call it a tidal wave, and suddenly this is one Big Thing that no-one wants.

You can add other adjectives to the tsunami description list. Try fast, try huge. But also try deceptive. For the wave that might tower 30 meters high when it strike the shores of Japan, Hawaii or Indonesia may have passed under countless boats in the deep ocean as barely a ripple. Like many awesome natural phenomena, it's all about the manner of their creation and the manifestation of their energy.

Tsunami (soo-nah-mee), a term adopted officially by scientists in 1963 to cover what previously had the far less memorable tag of seismic sea waves (calling them tidal waves is incorrect), translates from the Japanese as "harbor wave."

Like the terrifying Indian Ocean tsunami that killed thousands in 2004, they are most commonly caused by earthquakes beneath the ocean, though they can also be caused by underwater volcanic eruptions, landslides or, very rarely, asteroid impact – anything, in fact, that suddenly disturbs the equilibrium of the ocean on a large scale.

EARTHQUAKES, LANDSLIDES AND VOLCANOS

Oceanic earthquakes, for example, generate tsunami by deforming the seafloor, either suddenly raising or lowering it, an action that displaces the overlying water. In sub-marine landslides, the equilibrium is altered by the sudden movement of sediment on the sea floor. Above-water landslides unleash falling debris that disturb the water's equilibrium. When a marine volcano is the culprit, the force of the eruption causes the displacement of the water.NasaA NASA satellite captured this image of deep ocean tsunami waves about 30 to 40 kilometers from Sri Lanka, just hours after the quake that caused the 2004 tsunami. The image was made possible by sunlight reflected on the sea surface.

You can make tsunami-like waves in your bath, if you like. Rest the heel of your palm on the bottom of the bath and jerk your palm downward. Your movement beneath the surface makes waves on top of it. Imagine the earth doing something similar, but with billions of times more power. The energy of the ensuing wave is determined mainly by the amount of the seafloor deformation, but also by its velocity, the water depth near the earthquake source and the efficiency with which energy is transferred from the Earth to the water column above it. Add in a few big figures – like 4,000 meters as an average depth in the Pacific – and you are creating one hell of a wave.

Strangely, if you were far out at sea, you wouldn't even notice a tsunami hitting you. So long as there are thousands of feet of water to contain all that energy, a tsunami might only be about a meter or less tall as it sweeps under your boat, just another bit of swell. But let the water shoal as the wave nears land, and the monster emerges. Decreasing water depth slows the tsunami but if its speed falls, the laws of overall energy balance mean that Nature compensates by making the wave grow. When it finally reaches the shore, the tsunami has become a thing of terror, which can take various forms.

Reefs, bays, entrances to rivers, undersea features and the slope of the shoreline all play a part in how the tsunami strikes land. The single great towering breaking wave of popular image is actually one of the more rare manifestations. A series of breaking waves, a rapidly-changing tide or a bore (a step-like wave with a steep breaking front) are alternatives. At their worst, tsunamis can pile up a 30-meter high wall of water with which to attack the shore, and can flood hundreds of meters inland. A huge landslide that followed an earthquake in Lituya Bay in Alaska on July 9, 1958, created a freak tsunami from hell, sending a wall of water 525-meters high against the opposite shore of the bay. Miraculously, one boat out on the lake at the time survived.

90 RECORDED TSUNAMIS

From 1990 to 2005, there have been about 90 tsunamis recorded worldwide. Until the wave that devastated Southeast Asia, the top 10 of these had claimed just more than 4,000 lives, a figure now dwarfed forever. The worst of these previous killer waves, which struck Papua New Guinea's northern coast on July 17, 1998, is estimated to have claimed 2,200 lives.

Though both the incidence and death toll of tsunamis appears to be getting worse, this may be due to improved global communications and increases in coastal populations, rather than any real increase in these awesome events.

So where should you worry about tsunami? Well, the number and activity of tectonic plate joints around the Pacific Rim – known as the "Rim of Fire," for volcanic activity as well as earthquakes – makes Pacific coastal areas like those of Japan, Hawaii and Papua New Guinea places to be most wary. The rarity of tsunami in the Indian Ocean was one contributory factor to its impact, with countries in the region woefully unprepared for what happened.

Nor should Atlantic coast dwellers feel totally relaxed. While the Atlantic has far fewer tsunamis than the Pacific, the tectonic activity linked to the Puerto Rico Trench, the Antilles Subduction Zone around the eastern Caribbean and the South Sandwich Trench off South America has sent tsunamis to strike places like Puerto Rico and the Virgin Islands half a dozen times in recorded history, most recently in 1918 when 32 people died.

However, by far the most famous Atlantic tsunami actually struck southern Europe when, on November 1, 1755, a magnitude 8.6 earthquake hit the Gorridge Bank, a ridge off the coast of Portugal that was uplifted by the northward movement of the African Plate against the Eurasian Plate. The tsunami that resulted arrived in the Portugese capital of Lisbon just minutes after the quake, with at least three great waves around 10 meters high destroying much of the city. The waves also smashed against the coasts of Spain, North Africa, the Azores, Madeira and the Canary Islands, while minor damage occurred as far north as Ireland and as far west as the West Indies.

The speed at which tsunamis travel is one factor that makes them so deadly. Some experts calculate that roughly 90 percent of the fatalities usually occur within 150 miles of the source, as jet-liner-speed waves hit coastlines before people can escape. The failure to issue warnings in the Southeast Asia tragedies was a prime factor in the staggering death toll.

The vast reach of tsunamis is one of their most terrifying facts, causing waves to strike thousands of miles from where they were created. The rate at which a wave loses energy is related to its wavelength, and a wave with a huge wavelength like a tsunami loses very little energy as it travels. In very deep water, a tsunami will not only travel at great speed but will also travel vast distances with little energy loss. A tsunami generated off the coast of Chile in 1960, for example, was perfectly capable of killing 200 people in Japan, more than 14,000 miles away across the Pacific.

EARLY DETECTION AND WARNING

While there's little that can be done to protect coastal communities from being hit by tsunamis (earthquake-resistant housing is one thing, but surviving a 30-meter-high wall of water is another) there are advances now being made to improve early warning. Sea floor seismic sensors, which detect the initial earthquake, are sited off the coast of California and pressure gauges, which detect movements in water columns, are sited off the coast of Portugal. And there is still a place for land-based seismic detection.

On July 12, 1993, Japanese tsunami warning systems were tested by a magnitude 7.8 quake in the Sea of Japan. Five minutes after the main shock was detected, a warning of tsunami danger was broadcast on TV and radio, warning the endangered areas. Even this rapid response failed to beat the arrival of the first series of 20-meter waves, which struck the coastline nearest the source. For many of the 1,600 townsfolk of the fishing village of Aonae, however, it gave them precious time to flee to higher ground, minutes before a series of 10-meter tsunami destroyed hundreds of buildings. There were 200 people killed, but many lives were saved by the warning.

While the above devices, and others such as tidal gauges, can give warning of tsunamis relatively close to shore, a recent initiative in the United States targets those moving toward distant coastlines. The Deep Ocean Assessment and Reporting of Tsunamis project, commonly called DART, is based on a network of six deep-water reporting stations that use highly sensitive bottom pressure recorders to detect even minute increases in pressure.

The project can track tsunamis in real time far out at sea, but though they knew in advance where the SE Asian waves were heading the inability to raise a warning in threatened countries is one of the major points needing investigation after the tragedy of December 2004.