Sep 17, 2019 7:00 AM

James Cameron, Victor Vescovo, and the Saga of the Deepest* Solo Dive Ever

Vescovo says he dove deeper than Cameron. Cameron says not so fast. Perhaps only Poseidon knows for sure.

I’m not going to sit here and tell you the Challenger Deep, in the western Pacific Ocean, isn’t the deepest place on Earth. What I am going to tell you is that at the moment, only Poseidon himself can tell you exactly how deep it is, and that the measurement is such a sticky subject that James Cameron found time while shooting the Avatar sequels in New Zealand last week to set the record straight for me.

This saga begins in the year 2012, when Cameron crammed himself into a personal submarine and dove nearly 7 miles down into the Mariana Trench, off the coast of Guam. What he found was not a craggy sea floor teeming with life, but a beige, featureless plain of sediment, laid flat over hidden rock. “I was struck by how lunar it was,” he says. “But even the moon has contours.”

Cameron scooted along the bottom for three hours, taking in the otherworldly sights. After he returned to the surface, the Challenger Deep once again went black until this past May, when adventurer and rich person Victor Vescovo followed Cameron into the pit of the Earth. He was on his so-called Five Deeps Expedition, personally visiting the deepest point in all five oceans in a custom sub. The newcomer’s measurement of 10,924 meters set a record—beating the original trip down in 1960 by Don Walsh and Jacques Piccard, as well as Cameron’s 10,908—as the attending press release noted last week after Vescovo completed his final dive (the Molloy Deep in the Arctic Ocean).

And that’s when Cameron’s people got in touch to talk numbers, specifically those 16 discrepant meters. "I question that result," Cameron says. "I also question why nobody else has questioned that result."

Stand yourself against a wall, put a ruler on top of your head, and measure the ruler’s distance from the ground and you get a nice, objective number. You know just how many inches tall you are. It may vary a bit from day to day, depending on how long your hair is or if you’re wearing socks, but you’re getting a direct measurement in the known, physical universe. The certainty is downright satisfying.

The ocean provides no such satisfaction. If you wanted, you could drop an 11,000-meter cable down into the Challenger Deep and measure depth that way, but the thing would be buffeted by 7 miles of currents, obliterating any pursuit of accuracy.

Instead, scientists and explorers typically rely on sound or pressure—or both—to measure depth. Pressure, of course, increases as you go deeper. “Pressure is probably the best way to get an absolute measure of depth,” says Mark Zumberge, a research geophysicist at the Scripps Institution of Oceanography. But that alone won’t suffice, because water pressure can fluctuate as you descend—it depends in part on the water’s density, which changes up and down the water column based on temperature and salinity.

Illustration: Five Deeps Expedition

“To convert pressure to depth, you need to know the water density over the full water column and also the local value of gravity, which varies by about half a percent over the surface of the Earth,” Zumberge says. And if you’re trying to be really precise, it’s worth noting that gravity “even varies by a couple hundredths of a percent from the sea surface to the bottom of the ocean.”

The other way to measure depth is using sonar, but that comes with its own complications. The idea is to ping the sea floor with sound while timing how long it takes for the signal to get back to your boat. You have to know the temperature along the path to get an accurate reading, because sound travels faster through warmer water. Plus, if the sea floor is covered in sediment, as with the Challenger Deep, the ping might pierce that muck and end up bouncing off rock.

Either way, a measurement will have a margin of error of at least several meters, if not more. “The problem is, it’s counterintuitive for the average person, because we live in a day and age now where, through GPS, we kind of know where everything is,” Cameron says. “We know where we are, we know where our car is, we know where our phone is within a meter. So we're pretty darn spoiled.”

Both Cameron and Vescovo used pressure to calculate the depth of their dives. But while they visited the same general area of the Mariana Trench, they agree they were exploring different spots. Cameron says it was a plain, but Vescovo found features with sonar. “We were making real-time maps of the bottom of the Challenger Deep a week before we even dove there, mapping the heck out of it with multiple sweeps, and that's something James Cameron did not do,” says Vescovo. “To say the entire feature is completely and utterly flat as a billiard table, I don't think anyone can know that for sure. What I do know is our sonar map shows about a half-kilometer by half-kilometer area that there's definitely, like, a little bowl, that's what we called it, and then we dove there, and that's what all of our pressure sensors indicated.”

The two explorers also acknowledge that their depth measurements’ margins of error actually overlap. “Based on pure math, could we have dived the same depth that James Cameron did?” asks Vescovo. “Within the bounds of statistical analysis, it says of course there is, but it's not likely. But we're splitting hairs when we're talking about 10 or 15 meters out of almost 11,000.”

The media, though, latched onto the narrative of Vescovo beating Cameron’s record, largely because, well, Vescovo’s press release perpetuated that narrative. But all this talk of records misses the critical context of the tricky science of determining depth.

Cameron and Vescovo both insist that the bigger point is how underfunded and under-appreciated the ocean’s deepest depths are. The fact that Vescovo's team managed four dives into the Challenger Deep also demonstrates that a custom-built submersible can operate reliably in the brutal conditions 7 miles down.

And for Vescovo to reach the deepest point in all the world’s oceans is an incredible accomplishment, says Woods Hole Oceanographic Institution engineer Andy Bowen, who collaborated with Cameron. “Whether each of those dives achieved the record-setting number, I think, is far less important than the idea that humans have now demonstrated their ability to reach any part of the ocean with a submersible that can bring back real, meaningful improvements in our understanding of the deep ocean,” he adds.

Still, there’s something unsettling about not knowing exactly how far down the Challenger Deep is. We humans crave certainty, and it’s easy to assume that such things are easily knowable. But even if they were, those numbers could still shift unexpectedly. Given its seismic activity, the sea floor itself could have changed in the seven years between the two explorers’ dives. Today’s measurement could be tomorrow’s old news.

“Science is a process of refinement, and every bit builds on what went before,” Cameron says. “Nothing is ever set in stone in science—Newton had it all figured out until Einstein came along.”

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