Sail roughly 200 miles southwest of Guam and the seafloor drops away into a crescent-shaped scar so deep that Mount Everest could be lowered into it and still leave more than a mile of water above the summit. That scar is the Mariana Trench, and its lowest point, known as Challenger Deep, marks the deepest place anywhere in the world’s oceans. Getting there has required some of the most specialized engineering ever built for exploration, and even now, more people have walked on the Moon than have reached the bottom of this single trench. The gap between how well mapped the Moon’s surface is and how little of the deep ocean floor has been directly observed remains one of the more striking facts in modern exploration.
A Depth of Roughly 36,000 Feet
Challenger Deep bottoms out at approximately 36,000 feet, or close to 11,000 meters, below the ocean’s surface. That works out to nearly seven miles of water stacked directly overhead, a depth so extreme that if Everest were dropped in whole, its peak would still sit roughly 7,000 feet underwater. The trench itself stretches for hundreds of miles along the edge of the Pacific and Mariana tectonic plates, but Challenger Deep occupies only a small, narrow section near its southern end. The trench formed where the denser Pacific plate slides, or subducts, beneath the smaller Mariana plate, a process that over millions of years carved out one of the most dramatic depth changes found anywhere on the planet’s crust.
Found by a Weighted Rope in 1875
The first measurement of the trench came from the British Royal Navy’s HMS Challenger expedition, which lowered a weighted sounding rope into the depths and recorded a reading of 4,475 fathoms, or roughly 8,184 meters, a figure documented on the trench’s reference page. That figure, remarkable for the technology available at the time, undershot the true depth by more than two kilometers, but it correctly identified the region as unusually deep and gave the trench its name. More precise readings using sonar and pressure sensors would not arrive until well into the twentieth century, gradually revealing just how far short that original rope-and-weight measurement had fallen. Sonar-based depth soundings, which time how long a sound pulse takes to bounce off the seafloor and return, eventually replaced weighted ropes across oceanography entirely, cutting survey time from hours per reading down to a matter of seconds.
Pressure That Would Crush an Unprotected Submarine
At the bottom of Challenger Deep, atmospheric pressure exceeds 16,000 pounds per square inch, more than a thousand times the pressure at sea level. That crushing force is why only a handful of purpose-built vehicles have ever survived a descent, each requiring a pressure hull engineered to withstand forces that would instantly implode an ordinary submarine hull. The bathyscaphe Trieste made the first crewed descent in 1960, carrying two people to the bottom for a brief stay before returning to the surface, and it would be more than half a century before another crewed vehicle repeated the feat.
A Protected Monument Larger Than Most National Parks
The waters and seafloor surrounding the trench fall within the Mariana Trench Marine National Monument, a federally protected area that the NOAA Fisheries service describes as encompassing volcanic features, undersea mud volcanoes, and hydrothermal vents where unusual life forms survive in some of the most extreme conditions on the planet. Those vent ecosystems, powered by chemical energy rather than sunlight, have drawn comparisons to the kind of environments that might support life on icy moons elsewhere in the solar system, making the trench as much a target for astrobiology research as for pure oceanography. Amphipods, translucent shrimp-like scavengers, and other specialized organisms have been recovered from the trench floor, showing that even the harshest, highest-pressure environment in the ocean still supports a functioning food web rather than sitting entirely lifeless.
Sound Still Travels to a Place Light Never Reaches
No sunlight penetrates anywhere near Challenger Deep, but the trench floor is far from silent. Researchers who lowered a titanium-encased hydrophone to record ambient noise at the bottom found a steady mix of natural and human-made sound, according to a NOAA media release describing the recordings. Earthquake rumbles, the calls of baleen whales, and even the drone of ship propellers from the surface, thousands of feet above, all registered on the instrument, showing that sound waves travel through the deep ocean far more efficiently than light or radio signals ever could.
Only a Few Descents in More Than Sixty Years
Despite decades of ocean exploration, crewed visits to Challenger Deep remain exceptionally rare, with only a small number of piloted descents completed since the Trieste’s original 1960 dive. Filmmaker and explorer James Cameron piloted a solo submersible to the bottom in 2012, and private expeditions using newer submersibles have since added a handful more descents, gradually building a small catalog of firsthand observations from a place that remains harder to visit than low Earth orbit for most of human history. Each successful trip adds incrementally to what scientists know about a trench that still holds more mystery than most places on the surface of the planet. Modern expeditions increasingly pair crewed or robotic descents with sensors that record water chemistry, temperature, and biological samples along the way, turning each rare trip into as much a data-gathering mission as a feat of engineering endurance.
This article was produced with the assistance of AI and reviewed by Morning Overview editors.
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