The deepest point in the ocean could swallow Mount Everest with a mile to spare

Turn Mount Everest upside down and drop it into the deepest known point of the ocean, and its summit would still sit more than a mile underwater. That single comparison captures just how extreme Challenger Deep really is, a slot in the seafloor southwest of the Mariana Islands that took an accidental sounding, a cracked submersible window, and more than a century of increasingly precise instruments to fully understand.

An Accidental Discovery by the HMS Challenger

The trench was found almost by chance. On March 23, 1875, the crew of the British survey ship HMS Challenger stopped to take a routine depth sounding southwest of Guam, and because light winds meant the captain could let the ship float instead of burning coal to hold position, the ordinary weighted rope kept descending far longer than expected, according to HISTORY. It finally hit bottom at roughly 4,475 fathoms, or nearly 27,000 feet, by far the deepest of the ship’s nearly 500 soundings on a four-year scientific circumnavigation that helped found modern oceanography. The crew, stunned by the reading, repeated the measurement and got the same result both times; the immense pressure at that depth crushed the thermometers attached to the sounding line. They named the spot Swire Deep, after the expedition’s navigating sub-lieutenant, without realizing they had found the deepest point on the planet.

Why Pinning Down the Exact Depth Is Still Difficult

Modern instruments have refined that original estimate considerably, though not perfectly. NOAA’s current figure, drawn from a 2021 study led by Cmdr. Sam Greenaway using 2020 survey data, puts Challenger Deep at about 35,876 feet, or 10,935 meters, deep, according to Live Science. That is roughly 7,000 feet, or 2,100 meters, deeper than Mount Everest’s 29,032-foot summit is tall. Greenaway told Live Science that measuring the trench remains genuinely hard: sound takes about 14 seconds to travel down to the seafloor and back, and shifts in salinity, temperature and pressure along the way limit a ship-based sonar reading to roughly 25 meters of vertical accuracy. A seafloor pressure sensor can sharpen that figure, but building one accurate enough to withstand pressure roughly 1,100 times that at the surface, equivalent to an elephant balanced on a human thumb, is its own engineering challenge. Other recent surveys have put the depth as deep as 10,994 meters, and researchers interviewed by HISTORY note the trench floor is muddy and prone to underwater avalanches, meaning the true depth may shift slightly over time.

Twenty Minutes at the Bottom: The 1960 Dive

Humans did not reach the bottom until January 23, 1960, when Swiss oceanographer Jacques Piccard and U.S. Navy Lieutenant Don Walsh piloted the bathyscaphe Trieste to the seafloor. A plastic viewing window cracked under the pressure during the descent, but the pair still managed to spend about 20 minutes at the bottom before returning to the surface. They reported seeing what looked like a flatfish resting on the seabed, an observation later scientific review has treated as almost certainly a misidentification, since no fish has since been confirmed living anywhere near that depth.

James Cameron’s Solo Return, and Who Followed

No one went back for more than five decades. On March 26, 2012, filmmaker James Cameron made a solo descent in the purpose-built submersible Deepsea Challenger, a dive Live Science lists at roughly 35,787 feet, or 10,908 meters. The trench then saw a wave of visitors starting in 2019, when businessman and explorer Victor Vescovo reached the seafloor as part of an expedition that also set the current record for the deepest confirmed dive. HISTORY reports that Don Walsh’s own son, Kelly, later made the same descent, as did astronaut Kathy Sullivan, who had already flown in orbit and walked in space before adding the ocean’s deepest point to her own résumé.

The Chemistry That Lets Anything Live Down There

Despite the crushing pressure, cold, and total darkness, Challenger Deep is not empty. Shrimp-like amphipods gather in large numbers around bait dropped to the seafloor, alongside translucent sea cucumbers, microbes, and xenophyophores, single-celled organisms that can grow as large as a human fist. Biologist Paul Yancey, writing for the Schmidt Ocean Institute during a research expedition to the trench system, found that hadal snailfish thrive in high densities between 7,000 and 7,500 meters but essentially disappear below about 8,400 meters, a pattern he attributes to a protective molecule called trimethylamine oxide, or TMAO, which shields cellular proteins from being crushed by pressure but which fish bodies appear unable to accumulate beyond a certain depth. Amphipods, by contrast, thrive below 8,000 meters, and Yancey’s research has found some of the deepest species carry not just high TMAO levels but additional protective compounds, including one that a separate team of Japanese scientists discovered doubles as a wood-digesting enzyme. That biochemistry, layered on top of a trench most people will only ever see compared to a mountain that towers over it, is what makes Challenger Deep a live research target rather than a settled fact of geography.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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