As a friend on a jet ski towed Alo Slebir into a towering gray monster of a wave near Half Moon Bay on Christmas Eve, the Santa Cruz surfer had no idea he might be riding into the record books.

But a jaw-dropping photo of Slebir’s relatively tiny form racing down a collapsing mountain of ice-cold Pacific Ocean water has gone viral, with knowledgeable big-wave surfers around the globe speculating that the wave was over 100 feet tall and that Slebir, at 23, has set a world record.

Blakeney Sanford/Big Wave Challenge

But did he? For all of their ferocity, big waves are among the most ephemeral phenomena in all of sports — they’re almost impossible to precisely rank.

“You can measure Mt. Everest every day of the week for 100 years, it never moves,” said Bill Sharp, who has spent decades orchestrating the world’s most rigorous big wave surfing awards. “But even the biggest wave lasts only for a few seconds, and then it’s gone forever.”

So anointing a new world record holder isn’t like handing someone a gold medal after winning an Olympic event, Sharp said. It’s more like “naming a new pope.”

Alo Slebir, left, hitches a ride on a jet ski into the surf at Mavericks Beach.

(Frank Quirarte)

Once a year, Sharp, a former editor of Surfing magazine, and a “conclave of wise men and women” get together in a closed room and study photos and videos submitted by surfers from around the world in an effort to determine who was lucky — or crazy — enough to ride the biggest wave captured on film.

Submissions can come from anywhere, but the most common spots are Half Moon Bay, home to the mighty break known as Mavericks; the North Shore of Maui, home to a crushing wave known as Jaws; and, in recent years, an otherwise nondescript fishing village on the west coast of Portugal called Nazaré, where an underwater canyon funnels huge winter swells from the North Atlantic directly onto shore.

The current world record was set in Nazaré in 2022 when a German named Sebastian Steudtner rode a wave later calculated to have been 86 feet high.

But how, exactly, does one make such a calculation?

A process known as photogrammetry, Sharp said. Its starts by knowing the size of the surfer in the photo and then carefully extrapolating the size of the wave based on how much bigger it is than the surfer. So if the person is 6 feet tall, and the wave is 10 times their size, it’s a 60-foot wave.

Simple, right? Not when there are world-class athletes, world-class egos and international bragging rights involved.

First, you can’t just take somebody’s word about how tall — or, in this case, how short — they are when a potential world record is on the line.

“It requires actually going to the person and measuring them very carefully,” Sharp said. And since nobody rides a wave standing upright, they have to be measured in a variety of stances. “I have this system where there’s this grid, an inch-by-inch grid, and it goes on a wall behind them.”

Sharp shoots video with a camera set up far enough away to eliminate any optical illusions. He has the surfer bend and crouch and mimic actual surfing postures until he gets a pose that closely resembles the way they were standing in the footage shot when they were on the actual wave.

“Obviously, you know, you’re scared and you’re full of adrenaline,” Alo Slebir said of his potentially world-record wave at Mavericks Beach, “but there was no hesitation.”

(Audrey Lambidakis)

“This is crucial,” Sharp said. If you get the surfer’s height wrong by an inch or two, that can lead to huge errors when you multiply to calculate the height of a wave that might have been 15 times as tall as the surfer.

Then, there’s the problem of measuring the wave itself. Pretty much everyone can look at a photo and agree where the top is, Sharp said. But the bottom can be deceiving.

Where, exactly, does the flat water in front of the wave end and the upward slope begin? The answer can depend on a lot of factors, including whether the image was shot from a sandy beach at sea level, a cliff above the bay, or a jet ski rising and falling with the rolling swells.

That’s when the “regional factionalism” kicks in, Sharp said with a laugh. The experts locked in a room poring over the evidence try to keep their personal biases in check, but there’s always “a loud chorus of people” outside lobbying for their hometown heroes.

And it’s not just about the bragging rights. Since Nazaré was “discovered” in 2011, it has become an international tourist destination, with a large contingent of big-wave surfers, media and fans descending on the town of about 10,000 people every winter.

“I know in my heart we’re doing the best we can,” Sharp said, “but someone always feels ripped off.”

Sharp, supported by surf industry sponsors, began issuing the biggest wave of the year awards in the late 1990s. The first time they declared a world record was in 2001, after Mike Parsons from San Clemente rode a 66-foot wave at Cortes Bank, a submerged island about 100 miles offshore of San Diego that creates some of the tallest and most terrifying waves on Earth.

A year later, they declared a new world record when a Brazilian named Carlos Burle rode a 68-foot wave at Mavericks.

But in the mid-2000s, Hawaiian big-wave surfer Garrett McNamara heard that a sunny off-the-beaten-path beach in Portugal, where families go bodyboarding with their kids in summer, turns ferocious during winter swells.

In November 2011, McNamara rode a 78-foot wave there, setting the world record. The three most recent world records have been set there, as well.

Still, surfing is notoriously turfy, and a lot of Californians are pretty possessive about their favorite sport.

So when a strong December storm off the coast of Japan sent eye-popping swells rippling across the Pacific, surfers in the Golden State took note. When those swells crossed the international dateline pointing directly at Half Moon Bay, word spread among big-wave junkies and the scramble was on.

Alessandro “Alo” Slebir is definitely a big-wave junkie. He graduated from college a couple of years ago and has been working in construction and odd jobs to pay the bills while he hunts the perfect wave.

He was in Maui with his surfing partner, Luca Padua, when they realized the swell from the Japan storm was gaining steam and heading straight for Mavericks.

“A lot of people thought we were crazy for leaving, because the conditions on Maui were so good,” Slebir said. “But there was just no question we were going back. The question was, how fast can we get there?”

When they hit the water in Half Moon Bay on the morning of Christmas Eve, the waves “were only about 15 feet,” Slebir said. That’s big enough to send average surfers paddling for the safety of shore, but it did nothing to justify the expense and hassle of racing home from Maui.

“For me, I’m happy to have ridden the biggest wave of my life,” Alo Slebir said of his Christmas Eve feat. “I’ll let other people decide what size it was.”

(Frank Quirarte)

Slebir and Padua had been studying the data from offshore ocean buoys, however, and they knew monsters were heading their way. By midafternoon, waves of a starkly different dimension started to stack up on the horizon. One had a “unique look,” Slebir said, noticeably bigger than the others.

Padua was on the jet ski, Slebir was in the water, and they both knew instinctively they had to go for it. “Obviously, you know, you’re scared and you’re full of adrenaline,” Slebir said, “but there was no hesitation.”

When they reached the wave, Slebir let go of the tow and started hurtling down the near-vertical face of the wave. There was no time to look back to see how big it was. His only sense of the size came from how unbelievably fast he was going.

Slebir figures he was doing 40 or 50 mph, but the wave was so huge it was sucking water up off the reef in front of him at nearly the same speed, so he felt like he was slowing down. That’s a sensation he’d experienced only once or twice before, so he knew he was in the barrel of something so big it could kill him.

“Don’t fall,” he remembers thinking.

Frank Quirarte, who has been driving rescue jet skis for big-wave surfers at Mavericks and other legendary breaks for three decades, snapped the now-iconic photo of Slebir on the Christmas Eve wave. It went viral on Instagram, launching a tidal wave of excitement over the possibility — for many the near certainty — that the world record could be returning to California.

“It’s a big deal,” Quirarte said. The record “has sat in Portugal for the last how many years? A lot of people want it to come back.”

For now, all they can do is wait. The next conclave meets in Newport Beach in September. And with nine months to go, Sharp said, anything could happen.

In an interview Friday, Slebir seemed nonchalant about all the fuss. He noted that big-wave surfers a generation older spent decades riding giant swells without the benefit of cellphone cameras, or someone filming from the perfect spot at the perfect time.

“How many 100-foot waves have been ridden, but not documented?” he asked.

“For me, I’m happy to have ridden the biggest wave of my life,” Slebir said. “I’ll let other people decide what size it was.”