For many of the victims of yesterday's Indian Ocean tsunami, the first sign of the horror to come was not a visible wall of water speeding towards the beach. It was, in fact, quite the opposite.

Witnesses to the tidal wave that killed more than 20,000 people in eight South Asian countries described how the sea suddenly disappeared a few minutes before the wave hit: submerged rocks became visible, fish flapped harmlessly on the sand, small beaches suddenly stretched towards the horizon.

With no sophisticated early warning system in place to track the tsunami as it spread over the ocean, the drying-up of the sea was the only warning available that the tidal wave was coming. But the last major tsunami in the region was more than 60 years ago and few people understood its significance.

"The water went back, back, back, so far away, and everyone wondered what it was - a full moon or what?" said Katri Seppanen, a tourist on Phuket island's Patong beach in Thailand. "Then we saw the wave come, and we ran."

In the Pacific, an early-warning system already exists, based in Hawaii, that monitors submarine earthquakes, assesses the potential for a tidal wave and tracks any tsunamis as they ripple out to more than two dozen Pacific nations.

It is an expensive system, needing not just seismic monitoring but multiple sensors over the entire Pacific region and buoys to measure the progress of the wave.

Dr David Booth, a seismologist with the British Geological Survey, said today that such a system would have saved many thousands of lives in Indonesia, Sri Lanka, Thailand and India yesterday. The tidal wave took some two and a half hours to hit the coast of India and Sri Lanka, but no warnings were given.

But without such a system, nature's own warning should have been heeded.

"The most basic level of warning is probably the most effective," Dr Booth told Times Online. "If people are on the coastline, if the sea recedes quickly from the coast, than that is the precursor to a tsunami. That's when they should make for safety - they shouldn't wait for a warning from the local authority."

The disappearing beach phenomenon is simple enough to explain. A tsunami wave can travel thousands of miles across an ocean without losing power (one in 1960 crossed the Pacific after an earthquake in Chile and killed 200 people in Japan, more than 10,000 miles away).

When it hits shallow water off a coast it slows right down and builds in height, creating a vacuum that sucks the water off the very beaches it is heading towards.

There have been multiple aftershocks since yesterday's earthquake, which at 9.0 on the open-ended Richter Scale was the world's most powerful for 40 years, ripping open the ocean floor off the Indian Ocean island of Sumatra with a power calculated at a million times that of either of the atomic bombs dropped on Japan in 1945 and displacing billions of tonnes of water.

Dr Booth said there was little chance that any aftershocks in the same area would be powerful enough to cause another tsunami, but he said the same faultline, running down past the Indonesian island of Sumatra had the potential to cause another major earthquake to the south in the "months or years" to come.

He said that quake could affect not just the heavily populated coastal areas of the Indian Ocean but send a tsunami to the southwest, towards the northern coast of Australia.

John Howard, the Australian Prime Minister, said today that Australia would investigate taking the lead role in creating an Indian ocean tsunami warning system - although officials said it would take at least a year to set one up.

Phil McFadden, chief scientist with the government-funded Geoscience Australia, said that places close to the epicentre of the earthquake would have been hit so quickly that any warning would have come too late.

But if there had been a Pacific-style alert system covering the Indian Ocean, "there would have been time for people in Sri Lanka, across in the Maldives or somewhere like that to have done something about it".

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