Two people were killed when freak waves more than eight metres high hit a cruise ship in the Mediterranean. So what causes rogue waves, and where do they come from?
Bryony Jones

5 Mar 2010 - 11:39 AM  UPDATED 23 Aug 2013 - 2:09 PM

Two people were killed when freak waves more than eight metres high hit a cruise ship in the Mediterranean this week. So what causes rogue waves, and where do they come from?

Freak waves were once considered the stuff of seafaring legend - tall tales invented by sailors and used to explain away the loss of ships and even low-flying aircraft.

However, they are now recognised as a natural phenomenon.

Most common on the open ocean, rogue waves have also been recorded on more sheltered seas - such as the Mediterranean - and even on large lakes.

Rogue waves were only recognised as a real occurrence in 1995, after one 26 metres high hit an oil rig in the North Sea, and was recorded on the platform's monitoring devices.

They have since been monitored from space, with satellites recording waves up to 25 metres high far more frequently than had been expected.

The standard definition of a freak wave is one three times the size of those happening in the area at the time - 'normal' waves in the region of the 1995 oil rig wave were 12 metres high.

'Wall of water'

Those who have experienced them describe rogue waves as being like solid walls of water.

Ronald Warwick, captain of luxury liner the QEII when it came face to face with a 29-metre rogue wave during a hurricane in the Atlantic in 1995, said: "it looked as if we were going into the white cliffs of Dover".

The waves' causes are still under investigation, and their unpredictable nature makes them difficult to study.

There are believed to be several different types of freak wave, from a single 'wall of water' to a series of three peaks - known as 'three sisters' - like those which hit the Louis Majesty in the Mediterranean this week.

Experts say the waves are almost always generated by storm-related winds - but that they appear to come 'out of nowhere' to the shock of those who experience them.

"They always come when you are least expecting it," said Michel Olagnon, a specialist on the phenomenon at the French Sea Institute (Ifremer) in Paris.

Amplification, where two or more rogue waves overlap, can cause even greater damage.

Waves piling up

"As wind increases in intensity, it is first going to create relatively small waves, and then bigger ones, which travel faster," said Christain Kharif, a French oceanographer and co-author of Rogue Waves in the Ocean.

"Eventually the big ones will catch up, and the energy is concentrated as the waves pile up," he explained.

Tsunamis, which are sparked by seismic events such as earthquakes and volcanoes, are not the same as rogue waves.

While rogue waves have been blamed for damaging and even destroying ships at sea, tsunamis do most damage when they hit land, overwhelming coastal towns and cities.

Tidal waves, like the UK's Severn Bore, are a different phenomena again.

These occur on estuaries and rivers, when the incoming tide forms a wave - or bore - which rushes in, against the current of the river.