Ocean trenches are long, narrow depressions on the seafloor. These chasms are the deepest parts of the ocean—and some of the deepest natural spots on Earth. Ocean trenches are found in every ocean basin on the planet, although the deepest ocean trenches ring the Pacific as part of the so-called “Ring of Fire” that also includes active volcanoes and earthquake zones.
 
Ocean trenches are a result of tectonic activity, which describes the movement of the Earth’s lithosphere. In particular, ocean trenches are a feature of convergent plate boundaries, where two or more tectonic plates meet. At many convergent plate boundaries, dense lithosphere melts or slides beneath less-dense lithosphere in a process called subduction, creating a trench.
 
Ocean trenches occupy the deepest layer of the ocean, the hadalpelagic zone. The intense pressure, lack of sunlight, and frigid temperatures of the hadalpelagic zone make ocean trenches some of the most unique habitats on Earth.
 
How Ocean Trenches Form
 
Subduction Zones
When the leading edge of a dense tectonic plate meets the leading edge of a less-dense plate, the denser plate bends downward. This place where the denser plate subducts is called a subduction zone. 
 
Oceanic subduction zones almost always feature a small hill preceding the ocean trench itself. This hill, called the outer trench swell, marks the region where the subducting plate begins to buckle and fall beneath the more buoyant plate.
 
Some ocean trenches are formed by subduction between a plate carrying continental crust and a plate carrying oceanic crust. Continental crust is always much more buoyant than oceanic crust, and oceanic crust will always subduct. 
 
Ocean trenches formed by this continental-oceanic boundary are asymmetrical. On a trench’s outer slope (the oceanic side), the slope is gentle as the plate gradually bends into the trench. On the inner slope (continental side), the trench walls are much more steep. The types of rocks found in these ocean trenches are also asymmetrical. The oceanic side is dominated by thick sedimentary rocks, while the continental side generally has a more igneous and metamorphic composition.
 
Some of the most familiar ocean trenches are the result of this type of convergent plate boundary. The Peru-Chile Trench off the west coast of South America is formed by the oceanic crust of the Nazca plate subducting beneath the continental crust of the South American plate. The Ryukyu Trench, stretching out from southern Japan, is formed as the oceanic crust of the Philippine plate subducts beneath the continental crust of the Eurasian plate.
 
More rarely, ocean trenches can be formed when two plates carrying oceanic crust meet. The Mariana Trench, in the South Pacific Ocean, is formed as the mighty Pacific plate subducts beneath the smaller, less-dense Philippine plate. 
 
In a subduction zone, some of the molten material—the former seafloor—can rise through volcanoes located near the trench. The volcanoes often build volcanic arcs—island mountain ranges that lie parallel to the trench. The Aleutian Trench is formed where the Pacific plate subducts beneath the North American plate in the Arctic region between the U.S. state of Alaska and the Russian region of Siberia. The Aleutian Islands form a volcanic arc that swings out from the Alaskan Peninsula and just north of the Aleutian Trench.
 
Not all ocean trenches are in the Pacific, of course. The Puerto Rico Trench is a tectonically complex depression in part formed by the Lesser Antilles subduction zone. Here, the oceanic crust of the enormous North American plate (carrying the western Atlantic Ocean) is being subducted beneath the oceanic crust of the smaller Caribbean plate.
 
Accretionary Wedges
Accretionary wedges form at the bottom of ocean trenches created at some convergent plate boundaries. The rocks of an accretionary wedge are so deformed and fragmented they are known as melange—French for “mixture.”
 
Accretionary wedges form as sediments from the dense, subducting tectonic plate are scraped off onto the less-dense plate. Sediments often found in accretionary wedges include basalts from the deep oceanic lithosphere, sedimentary rocks from the seafloor, and even traces of continental crust drawn into the wedge. The most common type of continental crust found in accretionary wedges is volcanic material from islands on the overriding plate. 
 
Accretionary wedges are roughly shaped like a triangle with one angle pointing downward toward the trench. Because sediments are mostly scraped off from the subducting plate as it falls into the mantle, the youngest sediments are at the bottom of this triangle and the oldest are at the more flattened area above. This is the opposite of most rock formations, where geologists must dig deep to find older rocks.
 
Active accretionary wedges, such as those located near the mouths of rivers or glaciers, can actually fill the ocean trench on which they form. (Rivers and glaciers transport and deposit tons of sediment into the ocean.) This accreted material can not only fill trenches, but rise above sea level to create islands that “hide” the ocean trenches beneath. The Caribbean island of Barbados, for example, sits atop the ocean trench created as the South American plate subducts beneath the Caribbean plate.
 
Life in the Trenches
 
Ocean trenches are some of the most hostile habitats on Earth. Pressure is more than 1,000 times that on the surface, and the water temperature is just above freezing.  Perhaps most importantly, no sunlight penetrates the deepest ocean trenches, making photosynthesis impossible. 
 
Organisms that live in ocean trenches have evolved with unusual adaptations to thrive in these cold, dark canyons. Their behavior is a test of the so-called “visual interaction hypothesis,” which states that the greater an organism’s visibility, the more energy it must expend to catch prey or repel predators. In general, life in dark ocean trenches is isolated and slow-moving.
 
Pressure
Pressure at the bottom of the Challenger Deep, the deepest spot on Earth, is about 12,400 tons per square meter (8 tons per square inch). Large ocean animals, such as sharks and whales, cannot live at this crushing depth.
 
Many organisms that thrive in these high-pressure environments lack gas-filled organs, such as lungs. These organisms, many related to sea stars or jellies, are made mostly of water and gelatinous material that cannot be crushed as easily as lungs or bones. Many of these creatures navigate the depths well enough to even make a vertical migration of more than 1,000 meters (3,281 feet) from the bottom of the trench—every day.
 
Even the fish in deep trenches are gelatinous. Several species of bulb-headed snailfish, for example, dwell at the bottom of the Mariana Trench. The bodies of these fishes have been compared to tissue paper.
 
Dark and Deep
Shallower ocean trenches have less pressure, but may still fall outside the photic or sunlight zone, where light penetrates the water. 
 
Many fish species have adapted to life in these dark ocean trenches. Some use bioluminescence, meaning they produce their own “living light” in order to attract prey, find a mate, or repel a predator. Anglerfish, for instance, use a bioluminescent growth on the top of their heads (called an esca) to lure prey. The anglerfish then snaps up the little fish with its huge, toothy jaws.
 
Food Webs
Without photosynthesis, marine communities rely primarily on two unusual sources for nutrients. 
 
The first is “marine snow.” Marine snow is the continual fall of organic material from higher in the water column. Marine snow is mostly detritus, including excrement and the remains of dead organisms such as seaweed or fish. This nutrient-rich marine snow feeds such animals as sea cucumbers and vampire squid.
 
Another source of nutrients for ocean-trench food webs comes not from photosynthesis, but from chemosynthesis. Chemosynthesis is the process in which producers in the ocean trench, such as bacteria, convert chemical compounds into organic nutrients. The chemical compounds used in chemosynthesis are methane or carbon dioxide ejected from hydrothermal vents and cold seeps, which spew these toxic, hot gases and fluids into the frigid ocean water. One common animal that relies on chemosynthetic bacteria for food is the giant tube worm.
 
Exploring Trenches
 
Ocean trenches remain one of the most elusive and little-known marine habitats. Until the 1950s, many oceanographers thought that these trenches were unchanging environments nearly devoid of life. Even today, most research on ocean trenches has relied on seafloor samples and photographic expeditions. 
 
That is slowly changing as explorers delve into the deep—literally. The Challenger Deep, at the bottom of the Mariana Trench, lies deep in the Pacific Ocean near the island of Guam. Only three people have visited the Challenger Deep, the deepest ocean trench in the world: a joint French-American crew (Jacques Piccard and Don Walsh) in 1960 and National Geographic Explorer-in-Residence James Cameron in 2012. (Two other unmanned expeditions have also explored the Challenger Deep.)
 
Engineering submersibles to explore ocean trenches is presents a huge set of unique challenges. Submersibles must be incredibly strong and resilient to contend with strong ocean currents, no visibility, and intense pressure of the Mariana Trench. Engineering a submersible to safely transport people, as well as delicate equipment, is even more challenging. The sub that took Piccard and Walsh to the Challenger Deep, the remarkable Trieste, was an unusual vessel called a bathyscaphe.
 
The Deepsea Challenger, Cameron’s submersible, successfully addressed engineering challenges in innovative ways. To combat deep-sea currents, the sub was designed to spin slowly as it descended. Lights on the sub were not incandescent or fluorescent bulbs, but arrays of tiny LEDs that illuminated an area of about 30 meters (100 feet). To adapt to the pressure of the deep, the sub was shaped like a sphere—the walls of a square or cylinder-shaped vessel would need to be at least three times thicker to avoid being crushed. The sub’s fuel was augmented by seawater to prevent the oil from compressing. Perhaps most startlingly, the Deepsea Challenger itself was designed to compress. Cameron and his team created glass-based syntactic foam that allowed the vehicle to compress under the ocean’s pressure—the Deepsea Challenger came back to the surface 7.6 centimeters (3 inches) smaller than when it descended.
ocean trench
The Challenger Deep, part of the South Pacific's Mariana Trench, is the deepest part of the ocean.
Ocean Deep
Ocean trenches were not studied and explored until the 20th century. These deep-sea canyons were originally called “deeps,” such as the Challenger Deep or the Horizon Deep.
 
Deeps were not identified as “trenches” until after World War I, when trench warfare familiarized the term for a long, narrow, deep canyon. Today, the Challenger Deep is the deepest part of the Mariana trench, while the Horizon Deep is the deepest part of the Tonga trench.
Dive Deep
The Challenger Deep is 10,994 meters (36,070 feet) below the ocean’s surface. For comparison, Mount Everest, the world’s tallest mountain, is 8,850 meters (29,035 feet) above sea level. Mount Everest could fit inside the Mariana Trench with more than 2 kilometers (1 mile) to spare. 

Deep Disposal
The Challenger Deep is the deepest part of the ocean. It sits on a subduction zone, where the Pacific plate is subducting beneath the Philippine plate.

Some scientists argue that this makes the Challenger Deep the perfect place to dispose of toxic nuclear waste. The material would be far from human habitation and would melt into the Earth's molten mantle at the subduction zone. An international agreement (the London Convention) currently makes this proposed method of nuclear waste disposal illegal.

accrete
Verb

to build up or grow together.

accretionary wedge
Noun

mass of sediments scraped off from oceanic crust during subduction and piled up at the edge of the overriding plate. Also called an accretionary prism.

active volcano
Noun

volcano that has had a recorded eruption since the last glacial period, about 10,000 years ago.

Noun

a modification of an organism or its parts that makes it more fit for existence. An adaptation is passed from generation to generation.

Noun

region at Earth's extreme north, encompassed by the Arctic Circle.

asymmetric
Adjective

not identical on both sides.

augment
Verb

to enlarge or add to.

Plural Noun

(singular: bacterium) single-celled organisms found in every ecosystem on Earth.

basalt
Noun

type of dark volcanic rock.

Noun

a dip or depression in the surface of the land or ocean floor.

Noun

vehicle used to explore the deep ocean. Developed after the bathysphere.

Noun

light emitted by living things through chemical reactions in their bodies.

bone
adjective, noun

structure composing the skeleton of vertebrate animals.

buckle
Verb

to bend, fold, or fall apart quickly.

buoyant
Adjective

capable of floating.

Noun

deep, narrow valley with steep sides.

carbon dioxide
Noun

greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

chasm
Noun

a deep opening in the earth's surface.

chemosynthesis
Noun

process by which some microbes turn carbon dioxide and water into carbohydrates using energy obtained from inorganic chemical reactions.

Noun

edge of land along the sea or other large body of water.

cold seep
Noun

marine environment where hydrogen sulfide and methane seep up from beneath the seafloor and mix with the ocean water.

compound
Noun

substance having at least two chemical elements held together with chemical bonds.

compress
Verb

to press together in a smaller space.

contend
Verb

to sincerely assert.

continental crust
Noun

thick layer of Earth that sits beneath continents.

convergent plate boundary
Noun

area where two or more tectonic plates bump into each other. Also called a collision zone.

Noun

steady, predictable flow of fluid within a larger body of that fluid.

cylinder
Noun

tube or long, circular object.

deform
Verb

to put out of shape or distort.

delicate
Adjective

fragile or easily damaged.

delve
Verb

to research or investigate thoroughly.

dense
Adjective

having parts or molecules that are packed closely together.

depression
Noun

indentation or dip in the landscape.

descend
Verb

to go from a higher to a lower place.

detritus
Noun

non-living organic material, often decomposing.

devoid
Adjective

lacking or not having something.

dominate
Verb

to overpower or control.

earthquake
Noun

the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

eject
Verb

to get rid of or throw out.

elusive
Adjective

difficult to capture.

engineering
Noun

the art and science of building, maintaining, moving, and demolishing structures.

enormous
Adjective

very large.

equipment
Noun

tools and materials to perform a task or function.

esca
Noun

long, thin, fleshy growth from the head of an anglerfish.

evolve
Verb

to develop new characteristics based on adaptation and natural selection.

excrement
Noun

waste material discharged from the body.

expedition
Noun

journey with a specific purpose, such as exploration.

Explorer-in-Residence
Noun

pre-eminent explorers and scientists collaborating with the National Geographic Society to make groundbreaking discoveries that generate critical scientific information, conservation-related initiatives and compelling stories.

fluorescent
Noun

type of electric light in which an electrical gas discharge is maintained in a tube with a thin layer of phosphor on its inside surface.

Noun

all related food chains in an ecosystem. Also called a food cycle.

fragment
Noun

piece or part.

frigid
Adjective

very cold.

fuel
Noun

material that provides power or energy.

gas
Noun

state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

gelatinous
Adjective

resembling or behaving like a jelly, gel, or gelatin.

geologist
Noun

person who studies the physical formations of the Earth.

Noun

mass of ice that moves slowly over land.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

hadalpelagic zone
Noun

deepest zone of the open ocean, starting at around 6,000 meters (20,000 feet).

Noun

land that rises above its surroundings and has a rounded summit, usually less than 300 meters (1,000 feet).

hostile
Adjective

confrontational or unfriendly.

hydrothermal vent
Noun

opening on the seafloor that emits hot, mineral-rich solutions.

Noun

rock formed by the cooling of magma or lava.

illuminate
Verb

to shine light on.

incandescent
Adjective

a type of electric light in which light is produced by a filament heated by electric current.

inner slope
Noun

landward or continental side of an ocean trench.

innovative
Adjective

new, advanced, or original.

LED
Noun

(light emitting diode) device (semiconductor) that emits light when an electric current passes through it.

Noun

outer, solid portion of the Earth. Also called the geosphere.

lung
Noun

organ in an animal that is necessary for breathing.

lure
Noun

object used to attract an animal or other organism.

Noun

middle layer of the Earth, made of mostly solid rock.

marine
Adjective

having to do with the ocean.

marine snow
Noun

continuous fall of organic and inorganic particles (including the remains of marine organisms, fecal matter, shells, and sand) from the upper layers of the water column to the seafloor.

mate
Noun

one of a breeding pair of animals.

melange
Noun

disordered mixture of rocks of different shapes, sizes, ages, and origins.

metamorphic rock
Noun

rock that has transformed its chemical qualities from igneous or sedimentary.

methane
Noun

chemical compound that is the basic ingredient of natural gas.

Noun

movement of a group of people or animals from one place to another.

molten
Adjective

solid material turned to liquid by heat.

mountain range
Noun

series or chain of mountains that are close together.

Noun

place where a river empties its water. Usually rivers enter another body of water at their mouths.

navigate
Verb

to plan and direct the course of a journey.

Noun

substance an organism needs for energy, growth, and life.

oceanic crust
Noun

thin layer of the Earth that sits beneath ocean basins.

oceanographer
Noun

person who studies the ocean.

Noun

a long, deep depression in the ocean floor.

Noun

a long, deep depression in the ocean floor.

oil
Noun

fossil fuel formed from the remains of marine plants and animals. Also known as petroleum or crude oil.

organ
Noun

group of tissues that perform a specialized task.

organic
Adjective

composed of living or once-living material.

outer slope
Noun

oceanic side of an ocean trench.

outer trench swell
Noun

hill on the seafloor near an ocean ridge, where the oceanic lithosphere begins to subduct beneath the overriding plate.

parallel
Adjective

equal distance apart, and never meeting.

penetrate
Verb

to push through.

Noun

piece of land jutting into a body of water.

Noun

process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.

predator
Noun

animal that hunts other animals for food.

pressure
Noun

force pressed on an object by another object or condition, such as gravity.

prey
Noun

animal that is hunted and eaten by other animals.

producer
Noun

organism on the food chain that can produce its own energy and nutrients. Also called an autotroph.

rely
Verb

to depend on.

remarkable
Adjective

unusual and dramatic.

repel
Verb

to resist or push back.

resilient
Adjective

able to recover.

Noun

horseshoe-shaped string of volcanoes and earthquake sites around edges of the Pacific Ocean.

Noun

large stream of flowing fresh water.

rock
Noun

natural substance composed of solid mineral matter.

seafloor
Noun

surface layer of the bottom of the ocean.

Noun

base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

seaweed
Noun

marine algae. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria.

Noun

solid material transported and deposited by water, ice, and wind.

Noun

rock formed from fragments of other rocks or the remains of plants or animals.

Siberia
Noun

region of land stretching across Russia from the Ural Mountains to the Pacific Ocean.

spew
Verb

to eject or discharge violently.

sphere
Noun

round object.

startling
Adjective

surprising or astonishing.

steep
Adjective

extreme incline or decline.

subduction
Noun

process of one tectonic plate melting, sliding, or falling beneath another.

submersible
Noun

small submarine used for research and exploration.

sunlight zone
Noun

The upper zone of the ocean. This zone goes down to about 200 meters (660 feet). Also called the photic, euphotic, or epipelagic zone.

syntactic foam
Noun

material consisting of tiny hollow "microballoons" made from material such as glass or carbon.

tectonic activity
Noun

movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.

tectonic plate
Noun

massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

thrive
Verb

to develop and be successful.

tissue paper
Noun

very thin, translucent paper often used for wrapping delicate items.

toxic
Adjective

poisonous.

transport
Verb

to move material from one place to another.

unique
Adjective

one of a kind.

unmanned
Adjective

lacking the physical presence of a person.

vertical
Noun

up-down direction, or at a right angle to Earth and the horizon.

visibility
Noun

the ability to see or be seen with the unaided eye. Also called visual range.

visual interaction hypothesis
Noun

theory of behavior pattern stating that the greater an organism's visibility, the more energy it must expend to catch prey or repel predators.

volcanic arc
Noun

chain of volcanoes formed at a subduction zone.

water column
Noun

area reaching from the sediment of a body of water to its surface.