On March 23, 1875, the HMS Challenger stopped in the middle of the Pacific Ocean, southwest of the Mariana Islands, to conduct a routine depth sounding. As they’d done countless times before, the British scientists and sailors onboard lowered a weighted rope into the water. Unlike those other times, however, the rope kept falling, seemingly without end.

After nearly 27,000 feet, or about five miles, it finally hit bottom—the first human foray into what became known as the Mariana Trench.

As subsequent expeditions would reveal, the Mariana Trench actually plunges some 36,000 feet, or nearly seven miles, below the surface at its deepest point. That is deeper than Mount Everest is tall. At such depths, the water pressure is about 1,100 times greater than the pressure at the surface, equivalent to an African elephant standing on a person’s thumb, according to Mackenzie Gerringer, a deep-sea physiologist and ecologist at the State University of New York at Geneseo.

No light penetrates that deep, and the water temperature hovers close to freezing (though, as Gerringer points out, it’s slightly warmer at the bottom of the Mariana Trench than a little further up because the enormous pressure increases the amount of kinetic energy). “The Mariana Trench is one of the most famous and extreme environments on our planet,” researchers wrote in a 2020 issue of the Journal of Chemical Physics.

Formed tectonically, by the sliding of the Pacific Plate underneath the Philippine Sea Plate, the Mariana Trench stretches in an arc for over 1,500 miles through the western Pacific. Gerringer explains that most of the world’s other deep trenches are likewise in the Pacific, along the so-called “Ring of Fire.”

Until the H.M.S. Challenger expedition, no one knew the Mariana Trench was there.

A British warship, the HMS Challenger was refitted in 1872 with laboratories and storage space, and had most of its guns removed, at the urging of scientists from the Royal Society, who planned a worldwide voyage to study the ocean. Though some military tasks would be carried out, the British navy agreed that science would take precedent.

“Usually, it was the other way around, a navy expedition with a scientist or two onboard,” says Doug Macdougall, author of Endless Novelties of Extraordinary Interest: The Voyage of H.M.S. Challenger and the Birth of Modern Oceanography. He says the Challenger’s sailors “were instructed that their job was to serve the scientists. And I think some of the sailors were sometimes not too happy about that.”

On December 21, 1872, the Challenger set out from England with a crew of six scientists and around 250 sailors and officers. Running into a storm almost immediately, the ship headed to Portugal and the Canary Islands prior to crossing the Atlantic Ocean for the first time.

It later traversed the Atlantic a few more times, rounded the Cape of Good Hope, approached Antarctica before being stopped by ice, visited Australia, New Zealand, Hong Kong, the Philippines, New Guinea and Japan, crossed the Pacific to South America by way of Hawaii, and then rounded Cape Horn on its way home.

By the time the Challenger arrived back in England on May 24, 1876, it had covered nearly 70,000 nautical miles.

Every 200 miles or so, the Challenger’s scientists set up an observing station. Lowering a weighted hemp rope through a pulley, they determined the depth of the water at each station and took a sediment sample.

Generally, they also dredged the seafloor to capture organisms, record the temperature at various depths, collected water samples and measured the current.

The work could be difficult and dangerous. The rope broke the first time it was used, and the instruments attached to it were lost. Another time, in the Caribbean, a sailor was killed when “the tension on the dredge rope was so great that it pried an iron block from the deck and sent it flying across the ship,” explains Macdougall. In the Pacific, one of the scientists died of an infection.

In some ways, the Challenger’s scientists made ethically dubious decisions, at least by modern standards. For example, they at times used dynamite to collect fish specimens.

Nonetheless, the crew was able to amass great reams of data and “laid the foundation for the whole science of oceanography,” Macdougall says. Macdougall, a geologist who once worked analyzing lunar samples, calls the Challenger voyage the Apollo Program of its day, an expensive, government-funded science initiative that captured the public’s imagination.

The Challenger voyage discovered thousands of new species and proved that life could exist even at enormous depths. It uncovered cosmic dust, the remnants of comets and asteroids, in deep-sea sediment. And it dredged up polymetallic nodules, which, as Macdougall points out, “carpet the seafloor” in parts of the Pacific and are of interest to mining companies.

The Challenger also located the Mariana Trench, an event that came about unexpectedly. As the ship sailed toward Japan, it set up an observing station some 175 miles southwest of Guam, the southernmost of the Mariana Islands. Normally, the captain started the engines when data was being collected to hold the ship steady. But, with very light winds, he decided instead to save coal and float.

The weighted rope was lowered, turning up a measurement of around 4,475 fathoms (or nearly 27,000 feet). Of the nearly 500 depth sounding measurements taken aboard the Challenger at various locations around the world, this was by far the deepest.

“They were astonished,” Macdougall says of the scientists. “It wasn’t something they were anticipating at all.”

The experiment was repeated, and the results held. Both times, Macdougall says, the immense water pressure at such depths crushed the thermometers on the sounding rope. The scientists named the area “Swire Deep” after the Challenger’s navigating sub-lieutenant.

The Challenger’s scientists didn’t realize they were above a trench. Only much later would the full contours of the seafloor be mapped out. In 1899, a U.S. expedition lowered a rope more than 31,600 feet in that area. Then, in the early 1950s, a British navy vessel, the HMS Challenger II, used an echo sounder to record a depth of 35,640 feet.

Challenger Deep, as the deepest part of the Mariana Trench became known, has since been measured numerous times with single-beam sonar, multibeam sonar, side-scan sonar and pressure sensors. Though they differ slightly, all recent measurements put the depth of Challenger Deep at around 36,000 feet.

“It is muddy down there, and there are a lot of underwater avalanches at these sites,” Gerringer says. “So it is possible that the depth of the trench is changing.”

In 1960, Swiss oceanographer Jacques Piccard and U.S. Navy Lieutenant Don Walsh became the first humans to visit the ocean’s deepest place. Though a plastic window on their submersible cracked under the pressure, they managed to spend 20 minutes at the bottom.

No one else would enter Challenger Deep until 2012, when filmmaker James Cameron made a solo descent in a “vertical torpedo” sub. Since 2019, several other people have likewise reached the bottom of the Mariana Trench, including Walsh’s son, Kelly, astronaut Kathy Sullivan and adventurer Victor Vescovo.

Life populates even the extreme conditions of Challenger Deep. Shrimp-like amphipods can be “really abundant,” says Gerringer. If you put down bait, she says, “you’ll see hundreds of amphipods come to it very quickly.”

There are also translucent sea cucumbers, microbes and xenophyophores (single-celled organisms that can grow as large as a human fist). Fish, including hadal snailfish and cusk eels, live in the Mariana Trench as well, though not below around 27,000 feet. “We think that’s the upper limit of their pressure adaptation,” Gerringer says.

(In 1960, Piccard and Walsh claimed to see a flatfish in Challenger Deep, but scientists believe they were almost certainly mistaken.)

Gerringer has dived in submersibles to various trenches, including the Mariana Trench, and says she finds them “beautiful and peaceful.” It’s a far cry, she says, from the “monsters of the deep” myth that’s sometimes propagated.

She notes, however, that humans “are impacting these habitats, often even before we know what’s down there.” For instance, plastic bags have been observed, including one at the very bottom of the Mariana Trench, and deep-sea amphipods have been found to be ingesting microplastics. In addition, oxygen concentrations in the deep ocean are declining, which has been largely attributed to climate change.

“There is still a lot that we don’t know,” Gerringer says. Trench ecosystems, she adds, “are worth protecting.”