The tsunami wave would travel across the Pacific Ocean as fast as a passenger jet, yet would be barely noticeable until it approached the shoreline. Then it would devastate areas up to 300 metres above sea level, killing people and causing huge amounts of damage to buildings.
"The question is, can we forecast when it will happen? And the answer is, at present, no," said Dr Paul Segall, a geophysicist at Stanford University, California at the American Association for the Advancement of Science conference here.
But measurements taken by radar on board orbiting satellites have shown that on the south flank of the undersea Kilauea volcano, off Hawaii, there is a huge block 20 kilometres by 10km by 9km, which is moving at seven centimetres per year, forced along by lava flowing underneath it.
While that might not sound much, it creates the possibility of a "megaslump" - as the block crashes from one position to another, rather than sliding gently. That would cause an undersea earthquake equivalent in magnitude to 7 or 8 on the Richter Scale - which would prove devastating on land. Thousands of tonnes of water would be shocked into motion, and would head west across the Pacific. The tsunami would take 15 hours to arrive in Japan, where the effects would be dramatic, and it could take years for the country to recover.
While the technology exists to monitor the movement of the underwater block, there is still no way to predict whether the move - if and when it comes - will be sudden, prompting a "megaslump", or slower, like an underwater landslide. The latter would cause little damage.
One problem might be the public's reaction. In 1960, a tsunami hit Hilo in Hawaii: when the public was warned about it, many headed down to the beach to see it come in. Sixty people were killed.
Raindrops, bees and bacteria should have a price on their heads - and it would run into thousands of millions of pounds worldwide, American scientists said yesterday. According to new studies, the natural world provides humans with a pounds 20 trillion "service economy" which never shuts down or takes a holiday, yet we think is free. But we should put a price on it before we destroy it, said Gretchen Daily, of the biological sciences department at Stanford University, California.