A massive wave submerged a California beach. Was it a rogue wave?

Rogue waves were once just a spooky bit of marine folklore.

Sailors recounted tales of freakishly large waves that seemingly came out of nowhere, crashing into their ships — or completely submerging others, never to be seen again.

In the last 27 years, scientists have accepted that rogue waves are a real, scientific phenomenon. And when a massive wave hit a Ventura, Calif., beach on Thursday, many were quick to call it a rogue.

But was it? And what are rogue waves, anyway?

Rogue waves are a relatively rare phenomenon, but they can be dangerous. The largest one ever recorded, measuring 17.6 metres, was detected off the coast of B.C. by researchers at the University of Victoria in November 2020.

Those who study the ocean use a measure called significant wave height — which is the average of the largest 33 per cent of waves observed in the ocean — to determine if a wave is large enough to be considered a rogue.

"A rogue wave is a rogue if an individual wave is at least two times that significant wave height," said Scott Beatty, the CEO of coastal intelligence firm MarineLabs in Victoria, B.C.

In other words, rogue waves are usually more than twice the size of the waves around them, according to the U.S. National Ocean Service. 

The agency's website also says that the waves are "very unpredictable," often coming from unexpected directions that are inconsistent with the typical direction of winds and waters. 

Rogues might be called "freak" or "killer" waves, but scientists refer to them as "extreme storm waves."

They're often described as resembling walls of water, with steep sides and deep troughs.

"Rogue waves are often called rogue waves by accident, because people think just, 'I saw a big wave, therefore it must be a rogue,'" said Beatty.

However, rogue waves have a technical definition — and Beatty says it's hard to say whether the wave that crashed into a Ventura, Calif., beach on Thursday met that criteria.

A large individual wave, or set of waves, combined with a high tide might form something that looks like a rogue wave, Beatty explained, "but it is just indeed very large waves that end up running up along the beach and up and over barriers."