By BBC News Online's environment correspondent Alex Kirby
An immense wave could one day wreak havoc on the eastern seaboard of the US and elsewhere around the Atlantic.
Scientists say a volcanic eruption on the Canary Islands, off West Africa, could trigger a vast undersea landslide.
This would set off a tsunami wave capable of inundating coastal regions thousands of kilometers away.
But the disaster is unlikely to strike this century.
The warning comes from Dr Steven Ward, of the University of California, US, and Dr Simon Day, of the Benfield Greig Hazard Research Center at University College London, UK.
Writing in Geophysical Research Letters, they refine an earlier estimate of the likely consequences of the collapse of the western flank of the Cumbre Vieja volcano on the island of La Palma in the Canaries.
They believe a build-up of groundwater could destabilize a block of rock up to 500 cubic km in size, which could break off in a future eruption, rushing into the sea at up to 350 km an hour (220 mph).
The energy released by the collapse would equal the entire US electricity consumption for six months.
The dome of water it caused would be 900 meters (2,950 feet) high, and the resulting tsunami, higher than any in recorded history, would travel outwards, reaching speeds of 800 km an hour (500 mph).
Waves 100 m (330 ft) from crest to trough would strike the African coast, while north-eastwards they would affect Spain, Portugal and France, and could still be approaching 12 m (40 ft) when they hit the UK.
That is almost three times the maximum recorded after the Lisbon earthquake of 1755.
Dr Day said the waves striking the UK coast would not penetrate more than two or three kilometers inland.
But he told BBC News Online: "Weird things happen when tsunamis enter harbours or estuaries.
"If those resonate at a certain frequency, that may substantially increase the damage."
Across the Atlantic the damage would be far worse, with wave heights of more than 40 m (130 ft) expected in northern Brazil.
Dr Day said: "It's entirely possible you'd see 50-m waves coming ashore in Florida, New York, Boston, all the way up to Greenland, and in some cases reaching up to 10 km inland.
"And that would be about nine hours or more after the initial collapse."
The ensuing economic losses would probably be in trillions of dollars, even if there were enough warning to evacuate threatened areas and avoid massive loss of human life.
If the speed of the landslide proved to be 150 meters per second (490 feet per second), not the 100 m/s (330 ft/s) assumed, that could double the height of the waves reaching the US.
But Dr Day had some reassurance on the probable timescale of the catastrophe.
He said a collapse was unlikely this century, and perhaps for many centuries.
It could take several eruptions to destabilize the volcano enough to dislodge the western flank, and collapse would occur only after days or weeks of seismic activity.
He told BBC News Online: "We think you have to see some evidence of subterranean movement before there's a risk of collapse.
"The fact that we aren't seeing any movement gives us a lot of confidence Cumbre Vieja won't collapse spontaneously.
"But we've found that eruptions do tend to come in clusters. And there've been two in the recent past."
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It could happen also today.