Forget Rogue Waves, We Must Fear The Rogue Holes

For centuries, sailors have reported the existence of giant, freak waves, towering high above their compadres, more than twice the height of usual ocean waves. These accounts, like reports of mermaids and the kraken, were not taken too seriously. That might strike you as a little unfair, given they were describing a bigger version of a known phenomenon and not "a singing human woman with fish for legs", but until very recently, they were dismissed as if you'd said you'd just bumped into Aquaman.

According to the book Oceanography in the Days of Sail, Dumont d'Urville, whilst traversing the Indian Ocean in 1826 on board the ship Astrolabe, described seeing waves at least 24–30 meters (80-100 feet) tall, in a storm that claimed the life of one of the crew. At the time, the consensus was that it was not possible to reach heights over 9 meters (30 feet), but d'Urville reported what he saw anyway, given that it (plausibly) happened. Perhaps he thought that if waves truly can tower this high, people might want a heads up.

For his troubles, d'Urville was met with skepticism or outright mockery, with French mathematician and physicist François Arago making fun of the "truly prodigious waves with which the lively imagination of certain navigators delights in covering the seas".

It's unclear why people were so skeptical of such waves. Other large waves were reportedly encountered by Christopher Columbus's expedition in 1498, as well as steep troughs, while in 1861, a lighthouse in Ireland was damaged by a wave that seemingly reached the cliff top. But there are a few ideas why people were so unwilling to believe such accounts, including that it was due to ships back then being far more vulnerable than the steel ships of today.

"People who encountered 100-foot rogue waves generally weren’t coming back to tell people about it," Susan Casey, author of The Wave, explained to Salon. "And the people who did were thought to be exaggerating. 

In short, it could be a sort of reverse-survivor bias, where you don't believe in rogue waves because anybody who encountered one generally snuffed it.

On New Year's Day in 1995, that changed, as a 25.6-meter-tall (84-foot) wave smacked into the Draupner oil platform in the Norwegian North Sea, confirming their existence.

"It confirmed what seafarers had described for centuries," Francesco Fedele, associate professor at Georgia Tech's School of Civil and Environmental Engineering and lead author of a paper explaining how rogue waves are formed, explained in a statement at the time of the study's publication. "They always talked about these waves that appear suddenly and are very large – but for a long time, we thought this was just a myth."

We now know that tales of rogue waves, once mocked, were likely just "things that plausibly happened". But there is another phenomenon known as "rogue holes" that was also mocked, and that turned out to be real as well.

"Inverse formations, rogue wave holes have been mentioned [...] as deep troughs occurring before/after large crests," a study on the topic explains. "Thus, they usually accompany the elevated rogue waves."

ESA's MaxWave project, as well as observing numerous rogue waves, saw evidence of these rogue holes. But there is another type.

"On the other hand, they can also be observed as truly 'hole' rogue waves that are surrounded by the crests of equal size before and after the hole itself," the paper continues. "When the depth of such hole from trough to crest is much higher than the average height of the surrounding waves, it can equally be dangerous for navigation as the elevated rogue wave."

That study successfully proved the existence of such waves by reproducing them inside a water tank. According to the team, the phenomenon and rogue waves are not unrelated. The team used a carrier-envelope model to describe the rogue waves and holes they created, with a rapidly oscillating wave being the carrier, and the slowly varying function that modulates the amplitude being the envelope. 

"When the maximum of the envelope is at the point of the maximum of the carrier (ϕ = 0), the rogue wave appears as an elevated high-amplitude rogue wave," the team explains. "Conversely, when the maximum of the envelope coincides with the position of a minimum of the carrier (ϕ = π), the rogue wave appears as a deep hole on the surface of water."

Rogue holes are thought to be rare, though no less dangerous than their taller counterparts.