How deep is the deep end of the ocean? The answer turns on an array of factors nearly as wide as the sea itself: the barometric pressure over the site in question, the seawater’s density and temperature, the vagaries of measurement and, perhaps, on whether a world record is at stake.
The whereabouts of the planet’s deepest spot is widely agreed upon: the Challenger Deep, a muddy depression nearly seven miles down in a long fissure of the western Pacific, 200 miles southwest of Guam. The depression is estimated to be roughly a mile wide and seven miles long.
In April, Victor L. Vescovo, a rich investor who has skied the North Pole and climbed Mount Everest, piloted a submersible — part of a $48 million operation — into the Challenger Deep and declared his dive the deepest ever by a human. Global headlines followed. The record depth was given as 35,853 feet — a first by 52 feet, according to the team’s official statement.
Last month, Mr. Vescovo raised his profile further by making the final dive of what his team called “the world’s first manned expedition to the deepest point in each of the five oceans,” a list that includes the Challenger Deep. The Discovery Channel will feature the dives in a five-episode series.
Now, James Cameron is objecting. The creator of the films “Avatar” and “Titanic,” Mr. Cameron is a devotee of deep wrecks and recesses, including the Challenger Deep, which he explored in a 2012 dive that scrutinized the bottom for three hours. In a recent interview, he disputed Mr. Vescovo’s claim to history’s deepest dive.
“What he’s done is quite remarkable,” Mr. Cameron said of the businessman’s expedition. “Where I take exception is his saying he went deeper.”
Mr. Cameron likened the issue to an Everest climber claiming to have bested another mountaineer even though both reached the same summit.
“You can’t go deeper,” he said of the Challenger Deep. “It’s flat and featureless. So his gauge may read differently from mine, but he can’t say he’s gone deeper.” What’s irksome, Mr. Cameron added, “is that something can become part of the public record without substantiation.”
The interview was done at Mr. Cameron’s request and marks the first time he has publicly criticized Mr. Vescovo’s claim. He did so from Wellington, New Zealand, where he was filming an “Avatar” sequel. The initial email inquiry from an assistant read, “Request to Speak.”
The deep has long fascinated the mogul. He made “The Abyss” (1989), “Titanic” (1997) and documentaries about lost ships, including “Ghosts of the Abyss” (2003), which toured the disintegrating interior of the storied liner. He has personally made scores of deep-sea dives, including 33 to the Titanic, which rests more than two miles down on the North Atlantic seabed.
His 2012 dive into the Challenger Deep has been described as matching one conducted in 1960 by a pioneering team of two men, who were later honored at the White House. Mr. Cameron’s dive was the first solo effort.
The Challenger Deep was discovered by the H.M.S. Challenger, a British ship that sailed the globe from 1872 to 1876. Since then, many expeditions have sought to measure the fissure’s depth. The estimates vary by more than 500 feet. All measurements, whether made at sea, on land or in space, bear some degree of uncertainty.
Oceanographers say the uncertainty arises because the main tools of undersea measurement are sound waves. Sound can travel for hundreds of miles underwater. But the speed of sound varies with changes in salinity, temperature and pressure, complicating its use as a measuring tool.
That is especially so in the Challenger Deep. The sonic impulses from a surface ship must travel through many layers of seawater of differing composition before bouncing off the bottom and returning to the surface for analysis and interpretation. The path, by one measure, goes from warm to icy to warm again.
Physically sampling the layers can help in devising speed and depth corrections. Many oceanographers also use computer models that seek to compensate statistically for the uncertainties. Typically, depth readings are assigned a margin of error.
In 2014, four scientists at the University of New Hampshire reported on a recent Challenger measurement. They put the margin at plus or minus 25 meters, or 164 feet. Each depth measurement, they added, represents “at best an estimate.”
Such uncertainties could well undermine Mr. Vescovo’s claimed 52-foot margin of superiority. His own team implicitly raised questions when it estimated the margin of error for his record dive at plus or minus 10.5 meters, or just shy of 70 feet.
What about Mr. Cameron’s claim that the Challenger Deep is flat? His confidence arises not only from his own observations but from those of the Woods Hole Oceanographic Institution on Cape Cod, to which he donated the novel craft he piloted into the depression.
In 2009, well before Mr. Cameron’s 2012 dive, Woods Hole sent down an advanced robot to explore the famous deep. “It was like being on the Bonneville Salt Flats,” Andy Bowen, leader of the robot team, recalled in an interview, referring to the Utah desert known for extreme flatness.
Conceivably, Mr. Vescovo may have found a slightly deeper part, Mr. Bowen said: “Does it seem probable? Not really.”
Typically, depth measurements of the Challenger Deep are recalculated and often revised. The New Hampshire scientists told of a preliminary 2011 estimate that they were updating in their 2014 paper. The revised figure was 33 feet shallower.
In May, Mr. Vescovo’s team said it was still analyzing its April depth reading for the “Deepest Submarine Dive in History,” as the news release put it in a headline. The figure, the team conceded in a long footnote, might be “revised in the future.”
In an interview last week, Mr. Vescovo said the figure had indeed been revised to 35,840 feet, or 13 feet shallower.
But he denied Mr. Cameron’s charge that he had falsely claimed an exploratory first. He praised Mr. Cameron as a visionary pioneer of deep exploration, and noted that his own team had adopted some of Mr. Cameron’s technical innovations.
“I have enormous respect for him,” Mr. Vescovo said. “On this point, however, I scientifically disagree.” His expedition’s gear was far newer and more accurate at gauging oceanic depths than Mr. Cameron’s, he said, and had identified a slightly deeper area.
Mr. Vescovo spoke from Monaco, where he said he was visiting Prince Albert, a patron of ocean studies and exploration.
He volunteered that Mr. Cameron had raised the deepest-claim issue with him in an email, and said he planned to respond “in the next day or two.” He added that one way to resolve the question of whether one man dove deeper would be for both to submit their readings and supporting data to a panel of independent experts.
“Perhaps we will never know,” he said in a follow-up email. “All I can do is stand behind the actual data.”