Exploring Deep-ocean Trenches

Ocean trench
The Deep Discoverer ocean vessel exploring the Mariana Trench. It studied geological features similar to rocks and canyons found in the Alps and canyons in California. This was done during the 2016 Deepwater Exploration of the Marianas. NOAA Office of Ocean Exploration and Research.

There are places deep beneath the waves of our planet's oceans that remain mysterious and almost unexplored. Some are so deep that their bottoms are as far away from us as the upper reaches of our atmosphere. These regions are called the deep ocean trenches and if they were on a continent, they'd be deep jagged canyons. These dark, once-mysterious canyons plunge down as far as 11,000 meters (36,000 feet) into our planet's crust. That's so deep that if Mount Everest were placed at the bottom of the deepest trench, its rocky peak would be 1.6 kilometers beneath the waves of the Pacific Ocean.

Technically, tenches are long, narrow depressions on the seafloor. The harbor fantastic life forms not seen on the surface, animals and plants that thrive in the extreme conditions of the trenches. It has only been in the past few decades that humans could even consider venturing that deep to explore.

Mariana trench
A NASA mapping view of the Mariana Trench, which contains the Challenger Deep. NASA 

Why Do Ocean Trenches Exist?

Trenches are part of the seafloor topology that also contains volcanoes and mountain peaks higher than any on the continents. They form as a result of tectonic plate motions. The study of Earth science and tectonic plate motions, explains the factors in their formation, as well as the earthquakes and volcanic eruptions that occur both underwater and on land.

Deep layers of rock ride atop Earth's molten mantle layer. As they float along, these "plates" jostle up against each other. In many places around the planet, one plate dives under another. The boundary where they meet is where deep ocean trenches exist.

For example, the Mariana Trench, which lies beneath the Pacific Ocean near the Mariana island chain and not far from the coast of Japan, is the product of what's called "subduction." Beneath the trench, the Eurasian plate is sliding over a smaller one called the Philippine Plate, which is sinking into the mantle and melting. That combination of sinking and melting formed the Mariana Trench.

plates and ocean mapping
A combined image of Earth's plates, the plate boundaries, and ocean bottom mapping (called bathymetry).  NASA/Goddard Science Visualization Lab.

Finding Trenches

Ocean trenches exist in all the world's oceans. They include the Philippine Trench, Tonga Trench, the South Sandwich Trench, the Eurasian Basin and Malloy Deep, the Diamantina Trench, the Puerto Rican Trench, and the Mariana. Most (but not all) are directly related to subduction actions or plates moving apart, which take millions of years to occur. For example, the Diamantina Trench formed when Antarctica and Australia pulled apart many millions of years ago. That action cracked Earth's surface and the resulting fracture zone became the trench. Most of the deepest trenches are found in the Pacific Ocean, which overlies the so-called "Ring of Fire". That region gets the name due to tectonic activity that also spurs the formation of volcanic eruptions deep beneath the water.

The Challenger Deep in the Mariana Trench.
The Challenger Deep is part of the Mariana Trench in the South Pacific. This bathymetric map shows the deep in dark blue, along with surrounding underwater terrain. NASA/Goddard Visualization Lab 

The lowest part of the Mariana Trench is called the Challenger Deep and it makes up the southernmost part of the trench. It has been mapped by submersible craft as well as surface ships using sonar (a method that bounces sound pulses from the sea bottom and measures the length of time it takes for the signal to return). Not all trenches are as deep as the Mariana. Time seems to erase their existence. That's because, as they age, trenches are filled with sea-bottom sediments (sand, rock, mud, and dead creatures that float down from higher in the ocean). Older sections of the sea floor have deeper trenches, which happens because heavier rock tends to sink over time.

Exploring the Deeps

The fact that these deep-ocean trenches existed at all remained a secret until well into the 20th century. That's because there were no vessels that could explore those regions. Visiting them requires specialized submersible craft. These deep ocean canyons are extremely inhospitable to human life. Although people did send diving bells into the ocean prior to the middle of the last century, none went as deep as a trench. The pressure of the water at those depths would instantly kill a person, so no one dared venture into the deeps of the Mariana Trench until a safe vessel was designed and tested.

That changed in 1960 when two men descended in a bathyscaphe called the Trieste. In 2012 (52 years later) filmmaker and underwater explorer James Cameron (of Titanic film fame) ventured down in his Deepsea Challenger craft on the first solo trip to the bottom of the Mariana Trench. Most other deep-sea explorer vessels, such as Alvin (operated by the Woods Hole Oceanographic Institution in Massachusetts), do not dive nearly quite so far, but still can go down around 3,600 meters (around 12,000 feet).

The Weird Life in the Deep Ocean Trenches

Surprisingly, despite the high water pressure and cold temperatures that exist at the bottoms of trenches, life does flourish in those extreme environments. It ranges from tiny one-celled organisms to tubeworms and other bottom-growing plants and animals, to some very strangel-looking fish. In addition, the bottoms of many trenches are filled with volcanic vents, called "black smokers". These continually vent lava, heat, and chemicals into the deep sea. Far from being inhospitable, however, these vents supply much-needed nutrients for types of life called "extremophiles", which can survive in the alien conditions. 

Future Exploration of Deep Sea Trenches

Since the sea bottom in these regions remains largely underexplored, scientists are eager to find out what else is "down there." However, exploring the deep sea is expensive and difficult, even though the scientific and economic rewards are substantial. It's one thing to explore with robots, which will continue. But, human exploration (like Cameron's deep dive) is dangerous and costly. Future exploration will continue to rely (at least partially) on robotic probes, just as planetary scientists reply on them for the exploration of distant planets.

There are many reasons to keep studying the ocean depths; they remain the least-probed of Earth's environments and they may contain resources that will help people's health as well as a deeper understanding of the seabeds. Continued studies will also help scientists understand the actions of plate tectonics, and also reveal new life forms making themselves at home in some of the most inhospitable environments on the planet.


  • “Deepest Part of the Ocean.” Geology, geology.com/records/deepest-part-of-the-ocean.shtml.
  • “Ocean Floor Features.” National Oceanic and Atmospheric Administration, www.noaa.gov/resource-collections/ocean-floor-features.
  • “Ocean Trenches.” Woods Hole Oceanographic Institution, WHOI, www.whoi.edu/main/topic/trenches.
  • US Department of Commerce, and National Oceanic and Atmospheric Administration. “NOAA Ocean Explorer: Ambient Sound at Full Ocean Depth: Eavesdropping on the Challenger Deep.” 2016 Deepwater Exploration of the Marianas RSS, 7 Mar. 2016, oceanexplorer.noaa.gov/explorations/16challenger/welcome.html.


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Petersen, Carolyn Collins. "Exploring Deep-ocean Trenches." ThoughtCo, Aug. 28, 2020, thoughtco.com/ocean-trench-definition-4153016. Petersen, Carolyn Collins. (2020, August 28). Exploring Deep-ocean Trenches. Retrieved from https://www.thoughtco.com/ocean-trench-definition-4153016 Petersen, Carolyn Collins. "Exploring Deep-ocean Trenches." ThoughtCo. https://www.thoughtco.com/ocean-trench-definition-4153016 (accessed March 27, 2023).