Sailors are famous for their tall tales, and have long entertained the world with stories of ghost ships, sea monsters, mermaids and other wonders. Scientists are cautious about accepting such anecdotal evidence, and so accounts of giant waves taller than the mainmast (see FT177:40–41) tended to be taken with a pinch of sea salt. Experience shows that waves which look like 12-footers are often 6ft (1.8m) at most – cool judgement is difficult to exercise when your life is at stake. Victorian oceanographers calculated that any wave of more than 60ft (18m) would collapse under its own weight, and concluded that reports of anything larger must be exaggerations.

	Two large ships sink every week… but the cause is never studied to the same detail as an air crash

The stories of giant waves did have a couple of features in common though. One was that they were not associated with storms, but seemed to appear as a ‘wall of water’ behind a trough so deep it seemed to be a ‘hole in the sea’ in otherwise benign conditions. Another common factor was that the waves were not single but tended to come in small groups. They were not simply larger-than average waves in a storm, but freak waves outside the range of anything normally seen.

They have nothing to do with tsunamis or ‘tidal waves’, which are quite small out at sea, and only achieve large size close to shore, as they reach shallower water. There were some reliable accounts of monster waves, such as one measured at 112ft (34m) from the US Navy tanker USS Ramapo in 1934, and the immense wave that broke the bell on the Bishop Lighthouse in the Scilly Isles at 100ft (30m) above sea level. Clearly the 60ft rule was wrong, but freak waves were thought to occur at any given point only once every few thousand years and so were not thought to be significant. The situation didn’t change until research in the 1990s that prompted an EU-funded project called MaxWave (http: //w3g.gkss.de/projects/maxwave/) in 2000. This involved looking at some 30,000 radar images taken over a three-week period by ESA satellites, each covering an area of 5km by 10km (three by six miles). The study, which took four years, identified at least 10 waves with a height of more than 80ft (24m). This showed that such waves exist “in higher numbers than anyone expected”, according to Wolfgang Rosenthal, senior scientist at the GKSS Research Centre in Geesthacht, which analysed the data.

Modern ships are designed to cope with waves up to a height of 50ft (15m), the highest that was generally expected. The surprising frequency of rogue waves could account for mystery sinkings in which ships simply disappear without any warning. “Two large ships sink every week on average, but the cause is never studied to the same detail as an air crash,” says Rosenthal. “It simply gets put down to ‘bad weather.’”

When a huge wave strikes a ship, several things can happen. The ship may take on so much water that it sinks, or it may simply be overturned. If the wave strikes the ship end-on, one end will first dip down into the trough of the wave before being abruptly raised by the wave. In a large ship, this can produce forces strong enough to break the vessel’s back.

Between 1969 and 1994, giant waves are estimated to have sunk some 22 supercarriers, defined as ships with a length of 200m (656ft) or more, with the loss of over 500 lives. An unknown number of smaller ships will have suffered the same fate. Another side of MaxWave was to understand how such waves were formed. Computer modelling showed that outsize waves could be formed when slow-moving waves were caught up by a succession of faster waves moving at more than twice their speed. The two sets of waves merge, producing slower, larger waves. The model was confirmed by experiments in a giant wave tank in Hanover.

However, we cannot yet predict when and where such waves are likely. There are a number of physical processes that might cause them, and these depend on the winds, currents and geographic features in a particular place. Certain currents, for example, can focus waves as a lens focuses light. The next stage of research is WaveAtlas, which map out the incidence of giant waves across the world’s oceans and attempt to identify areas where they could be a hazard to shipping. The fi rst WaveAtlas results were presented last year.

From a fortean point of view, this is all quite remarkable. Rogue waves have gone smoothly from being ‘damned’ – rejected as impossible by the scientific establishment – to being accepted as entirely respectable. Generally with unexplained phenomena there is far more resistance before they are admitted to the mainstream. Nobody has suggested that the waves detected by radar are just an ‘imaging artefact,’ which is what has happened with the thousands of tiny comets striking the Earth’s atmosphere apparently detected on satellite imagery by Dr Louis Frank. Nor have there been suggestions that the wave tank simulation doesn’t necessarily match real life, as has happened with attempts to recreate ball lightning in the laboratory.

Perhaps rogue waves have been accepted with relative ease because lives were at risk if the denial continued. Or maybe it is because their existence opens up lucrative new areas for research. But it seems more likely that the breakthrough has occurred because no great reputations rested on their being impossible in the first place.