“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 and time 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 seven 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.