An extra-tall wave struck a cruise ship off the Mediterranean coast of Spain this week, claiming two lives and injuring one person on board. Though the wave may not qualify as a "rogue wave," it could have been created by the same forces.

To officially be rogue, the wave's height must be more than double the "significant wave height" of the area, which is calculated by averaging the height of the tallest third of all the nearby waves.

The wave measured 26 feet tall and shattered plate-glass windows at the bow of the vessel. Still, it wasn't very tall compared to some of the waves oceanographer Libe Washburn of UC Santa Barbara has seen.

"I was surprised it was really that damaged by a 26-foot-high wave," Washburn said. "Twenty-six feet isn't that big."

Until recently, scientists were skeptical that rogue waves even exist, because evidence of them was mostly anecdotal. More often called "freak waves," these monsters of the sea were confirmed only six years ago by satellite images and extensive studies carried out by MaxWave, a research group funded by the European Commission.

Waves over 100 feet tall have been spotted by oceanographers, scientists and vessel passengers. The highest wave ever recorded was 112 feet tall, spotted in the Pacific by a U.S. Navy tanker in the 1920s. Now, whenever large ships get lost at sea and never return, many are quick to speculate they were victims of rogue waves.

Rogue waves occur in the open ocean in a number of ways. One common cause is when two smaller waves coalesce to produce a very large wave for a short time.

"You get waves that add up – smaller waves that constructively interfere and for a short time produce a very large wave," Washburn said. "When they add up, they can make an extra high crest and an extra deep trough."

Another way rogue waves propagate is when an ocean wave encounters a very strong current that's running counter to the direction of the wave, according to Washburn. The Agulhas Current, which flows down the eastern coast of South Africa, is notorious for producing rogue waves.

"It's very dangerous at the Agulhas," Washburn said. "Even if you're on a big ship, that doesn't mean you're any safer."

Storm-related wind is a factor as well. Strong winds transfer energy into the waves, creating interactions between them. Large waves take energy from smaller ones, creating a bigger and bigger wave, said oceanographer Peter Challenor of the National Oceanography Centre in the United Kingdom, in an interview with Agence France-Presse.

Images: 1) NOAA. 2) NASA.