How Deep Under the Ocean Have Humans Explored?

Hugo Britt June 22, 2023

Welcome to Thomas Insights — every day, we publish the latest news and analysis to keep our readers up to date on what’s happening in industry. Sign up here to get the day’s top stories delivered straight to your inbox.

Image Credit: Oliver Denker / Shutterstock.com

The bottom of the Challenger Deep at the southern end of the crescent-shaped Mariana Trench isn’t the sort of place people most would choose to spend their vacation. At 35,853 feet at its deepest point, it’s a cold 34^oF, pitch dark, and exerts a pressure more than 1,071 times that at sea level.

Yet the world’s deepest place exerts a strong pull for explorers, similar in many ways to the magnetic attraction of Mount Everest. Speaking of which, if you were to drop Everest into the Challenger Deep, its peak would be submerged by more than 1.2 miles.

Undersea Exploration Methods Through History

Technology has come a long way since mariners first began sounding the depths with a weighted line. And recently, OceanGate's submarine, the Titan, has reignited discussions of the technology necessary for deep-sea exploration.

Sounding weight: Essentially a sinker on a cable, this instrument has been used for centuries by sailors to chart the depths of unknown waters and avoid running aground. By the 1800s, interest in deep-sea exploration led to the development of long submarine cables. British explorer Sir James Clark Ross used a sounding weight to survey to a depth of 12,139 feet in 1840, collecting jellyfish and worms at 6,500 feet.
Baille sounding machine: Similar to a sounding weight, this device included a tube that drove into the sediment on the seabed to allow the collection of specimens, including marine life. This was the main instrument used by the British laboratory ship HMS Challenger, which surveyed 68,927 nautical miles between 1872 and 1876, collected 4,700 new species of marine life, and provided the first exhaustive map of the deep ocean basins.
Echo-sounding: This determines the depth of water and maps the seabed using an acoustic echo or a pulse of sound sent from the ship to the seabed and back up to the ship.
Bathysphere: William Beebe and Otis Barton developed this manned steel submersible on the end of a cable that allowed them to reach a depth of 1,427 feet in 1930; 3,028 feet in 1934; and 4,495 feet in 1948. The submersible’s passengers faced the nightmarish prospect of the cable breaking and the bathysphere plunging to the depths to be crushed under pressure.
Bathyscaphe: Developed by Swiss physicist Auguste Piccard, it could withstand the pressure at 4,600 feet. Auguste’s son Jacques Piccard famously used an improved bathyscaphe (Trieste) to descend with U.S. Navy Lieutenant Donald Walsh to the bottom of the Challenger Deep, reaching 35,810 feet in 1960.
Manned submarines: These can take three people as deep as 11,811 feet, and come equipped with lights, cameras, and robotic arms for collecting samples. An example of these is the Alvin.
Unmanned submersibles: These originally resembled steel spheres (benthographs) equipped with lights and cameras in the 1950s. Today, scientists use remote operated vehicles (ROVs) piloted via a cable connected to the ship above. Autonomous underwater vehicles (AUVs) are robotic submarines that are programmed in advance rather than piloted during the mission.

Famous Expeditions to the World’s Deepest Point

Following Piccard and Walsh’s record-setting descent to the bottom of the Challenger Deep, other famous expeditions include:

2012: Filmmaker James Cameron, of Titanic and Avatar fame, completed the first solo mission to the bottom of the Challenger Deep in his vessel the Deepsea Challenger.
2019: Victor Vescovo reached a deeper part of Challenger Deep at 35,853 feet, breaking the record for the deepest dive in DSV Limiting Factor. His dive was part of the Five Deeps Expedition to reach the bottom of every ocean on Earth.
2020: Former NASA astronaut Kathryn Sullivan became the first woman to reach the deepest point in the ocean and the first person to travel both to space and to the Challenger Deep.
2022: Dawn Wright was the first Black person to descend to the Challenger Deep.

What’s Down There?

Geophysical researchers are interested in the chemical, biological, and physical conditions of the world’s deepest seabeds.

Victor Vescovo described the bottom of Challenger Deep as “a flat, beige basin covered with a thick layer of silt … chilly, and quiet.” He observed yellow and red rocky outcrops, which were likely bacterial mats or chemical deposits created by chemosynthetic microbes. Life at the bottom included creatures that were small and translucent: Mariana snailfish, supergiant amphipods (shell-less crustaceans), and sea cucumbers.

Sadly, the other thing to be found at the ocean’s deepest point is pollution. Vescovo found a plastic bag and candy wrappers, while past research has discovered elevated concentrations of PCBs (a chemical banned in the 1970s) in Mariana Trench crustaceans. Amphipods have been discovered with microplastics in their stomachs. Traces of carbon-14 from nuclear bomb testing have also been detected.