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What’s at the bottom of the Mariana Trench, and how do we know? How deep does the ocean go? We’ll answer these questions and more in this deep dive into the world’s oceans. Come on in: The water’s fine!
How Deep Is the Ocean?
The short answer is that it depends on where you look. Near the world’s coastlines, the ocean is just a few hundred meters deep because of the continental shelf, the area of shallow seafloor surrounding large landmasses. The continental shelf sometimes extends for hundreds of miles out to sea. Coral reefs, benthic life, and a huge variety of ocean fish take advantage of these shallow waters, nutrient-rich and warmed by the Sun. Within a hundred meters of the surface, there’s enough sunlight to support plant growth such as giant kelp.
Beyond the continental shelf, the seafloor takes on a steeper downward incline. This continental slope plunges into depths sunlight cannot reach, through the aphotic zone all the way to the gigantic abyssal plain. Much of the abyssal plain is between 3,000 and 6,000 meters (1.85 to 3.7 miles) down. It is the largest, flattest, deepest, and least explored region of the planet: the abyssal plain alone accounts for some 51% of Earth’s surface area.
As the continental shelf gives way to the ocean depths, trenches and canyons gouge into the Earth’s crust. Submarine canyons, underwater erosion, and drainage systems analogous to above-water canyons on Earth are a common feature of continental shelves across the world. Trenches are created by plate tectonics and they often run roughly parallel to the coastline of their continent. The deepest of them all is the Mariana Trench, off the coast of Japan. Challenger Deep, the lowest point in the deepest canyon on Earth, lies 35,876 feet below sea level at the southwestern extreme of the Mariana Trench (6.8 miles, or 10.9 kilometers).
How Many Oceans Are There?
Depending on when you grew up and where you live, you probably learned that there were either four or five oceans. The first four are the Arctic, Atlantic, Indian, and Pacific oceans, but the US Board on Geographic Names recognized a fifth ocean a bit more than 20 years ago. The Antarctic, or Southern ocean, is the last of the five named oceans. However, that name hasn’t been formally ratified by the International Hydrographic Organization, because scientists haven’t yet reached a consensus on the fifth ocean’s boundaries. (For example, where does the South Pacific end and the putative Antarctic Ocean start?)
So how many oceans are there? Four, five… or one.
Credit: Futurama
All of the bodies of water on Earth that qualify as named oceans are directly connected to each other. Technically, they can all be considered parts of one colossal world-spanning ocean. Ferdinand Magellan’s expedition managed to circumnavigate the planet by ship because Earth’s oceans all flow into one another. NOAA estimates that it takes roughly a thousand years for a given amount of water to circulate around the world via what’s known as the Global Conveyer Belt, and while that’s achingly slow by human timescales, it’s fairly quick compared with deep geologic time.
Meet the Spilhaus projection, an ocean-centric map of the world’s waters. The Coral Triangle is a region of shallow ocean corresponding to the continental shelf between Indonesia and Australia, highlighted in red below.
Credit: The Seattle Aquarium
We split the world into five oceans, for the same reason telling people you live on “Earth” is generally an ineffective way to send a letter. Hydrodynamically speaking, there’s a long-term argument for seeing the world as largely inundated by a single global ocean. Practically, however, it’s a lot easier to anchor around phrases like “North Atlantic” or “South Pacific” as opposed to “over in the biggest part.”
Oceans vs. Seas
What’s the difference between an ocean and a sea? Oceans on Earth are defined by several characteristics. First, an ocean (or the ocean) is a large body of salt water that exists within the deep depressions that characterize 70-71 percent of the world’s surface. The Dead Sea in Israel is over 1,400 feet below sea level, but the Challenger Deep is 35,876 feet down at its lowest point. While most of the ocean bottom is nowhere near that low, life on land lives on the metaphorical penthouse floor.
It’s not just saltwater that characterizes an ocean, though. In order to qualify as an ocean, a body of water cannot be mostly or entirely surrounded by land. The Black Sea and Mediterranean Seas are both almost entirely landlocked, with the Black Sea connecting to the Aegean, and the Aegean Sea (itself part of the Mediterranean) connecting to the global ocean through the Straits of Gibraltar. In comparison, the Spilhaus projection shows that the Earth’s land masses don’t all connect to one another, but its oceans form a gigantic, unbroken body of water that wholly encompasses the land.
Seas are still part of what we call the “World Ocean” (getting back to that annoying singular answer). But scientists sometimes delineate between oceans and seas because seas, with their shallower depths and proximity to land, are impacted by somewhat different processes than the depths of the ocean. The image below shows a bathymetric map of the ocean floor, where pink denotes the shallowest water (continental shelves and oceanic plateaus), compared with the yellow-green of mid-ocean ridges and the blue and purple depths of the abyssal plain.
Great swathes of the ocean floor are at basically the same elevation. However, thanks to plate tectonics, there’s more to the story.
Credit: National Oceanic and Atmospheric Administration
Another important characteristic of seas is that they tend to be shallower than oceans and often exist on the continental shelf. Every continent has a continental shelf that extends off its coast. But the size of the shelf and how quickly it descends towards the deepest parts of the ocean, known as the abyssal plain, varies from place to place.
Ridges, Trenches, and the Abyssal Plain
Early oceanographers thought the ocean bottom would almost universally resemble the abyssal plain, thanks to erosion depositing unfathomably large amounts of sediment into the ocean basin on deep time scales. Instead, we found an underwater landscape far more varied than expected.
The abyssal plains of the world’s ocean(s) show very little topographic difference. The smooth, flat abyssal plain resembles what the earliest oceanographers thought the entire ocean bottom looked like a few hundred years back. However, evidence that there was much more to the story began to arrive in the 1800s as the first successful oceanographic surveys returned to port. The advent of sonar allowed for the first in-depth surveys of the deep ocean floor, and these studies drove geologists to finally accept a radical theory proposed decades earlier and dismissed for being “eccentric, preposterous, and improbable,” according to the United States Geologic Survey: plate tectonics.
Credit: J. M. Watson/United States Geological Survey
The Pacific Ocean is riven with oceanic trenches 3-4 km deeper than the ocean bottom that they cut through. These trenches mark areas where continental plates collide in what are known as convergent plate boundaries. Trenches are where half the magic of plate tectonics happens—specifically, the part where crust from one plate plunges under a different plate, leading to long-term recycling of the planet’s surface crust. The other half of the magic happens at mid-oceanic ridges.
The British Challenger expedition detected the first evidence of a mid-oceanic ridge running down the Atlantic Ocean, but the ridge was initially thought to be unique to the Atlantic rather than a shared feature of oceans around the world. It wasn’t until the advent of sonar that scientists realized how universal these ridges were. Volcanism is a defining feature of mid-oceanic ridges and the oceanic crust immediately around the ridge is the youngest and lowest-density crust. Some ridges spread more slowly than others; slower-spreading ridges like the mid-oceanic ridge in the Atlantic are steeper than the faster-spreading ridges of the Pacific. Ridges thrust up from the abyssal plain by tectonic forces can reach thousands of meters in height, and run for hundreds or thousands of miles.
The discovery of both trenches and mid-ocean ridges paved the way for geologists to accept plate tectonics by providing an explanation for how the Earth’s crust is both created through seafloor spreading and destroyed via subduction. Before these discoveries, plate tectonics had foundered as a theory precisely because no one could provide a scientific explanation for how crust was recycled, transformed, or reshaped on an ongoing basis across geologic timescales.
The Mariana Trench
Part of the tectonic “Ring of Fire” encircling the Pacific Ocean, the Mariana Trench is a subduction fault where the older crustal rock of the Pacific continental plate is forced below the smaller Mariana plate to its west. The Pacific plate is cooler and denser than the Mariana plate, and as it is subducted below the Mariana plate, water trapped in the sinking rock causes the volcanic eruptions that created (and continue to change) the Mariana Islands.
Challenger Deep, the deepest of the Mariana Trench’s three “pools,” was named after the British expedition Challenger, the first to sound its fathomless depths with a weighted rope.
The Technology That Takes Us There
Deep ocean exploration began with the diving bell: a rigid structure that contains air, used to transport divers to an area for underwater work. It was known to Aristotle in the 4th century BC. From its humble origins as an underwater scrapper’s aid, salvage in antiquity as well as the present day, its invention and improvement were stepping stones on our way to more robust, deeper diving underwater exploratory vehicles.
As late as the 1920s, however, humans in diving suits were limited to dives of several hundred feet. Submarines of that era could reach lower depths but lacked windows. This was a concern for naturalist William Beebe, who wished to observe undersea wildlife in its natural habitat. The (cough) Venerable Beebe—he died in 1962 at age 85—built his Bathysphere in 1928 and 1929. On August 15, 1934, Beebe set a dive record of 3,028 feet (923 meters). Beebe initially wanted to build a cylindrical enclosure until he was talked into using a spherical shape by Otis Barton, who shared Beebe’s interest in underwater exploration.
The Bathysphere was followed by the Benthoscope in 1949. The Benthoscope was designed to reach even deeper than the Bathysphere had done and was built thicker with a higher crush depth (10,000 feet). Beebe’s 1949 dive set a still-standing record for the deepest any cable-suspended submersible has ever dived. The same year Beebe set his record, August Piccard debuted his bathyscaphe. A bathyscaphe is a free-diving, self-propelled vehicle and a more direct progenitor of modern submersibles than the towed Beebe designs.
The Trieste, piloted by Swiss oceanographer Jacques Piccard (son of Auguste) was a refined version of the earlier FNRS-2 bathyscaphe design that launched a few years earlier. Where the FNRS-2 was used in a series of unmanned dives, the Trieste was the first crewed submersible capable of reaching the very bottom of the global ocean. The Challenger Deep dive set a descent record of 35,814 feet (10,994 meters). The depth record Trieste set in 1960 would stand until 2019 when it was edged out by Victor Vescovo’s Limiting Factor at a depth of 35,843 feet.
Another important deep-ocean submersible is the DSV-2, aka Alvin. First commissioned in 1964, Alvin was designed as a more maneuverable successor to bathyscaphe designs like Trieste. Originally intended for a design depth of 8,010 feet (2440 meters), Alvin has been upgraded repeatedly throughout its operational lifetime. Alvin first came to the world’s attention in 1966, when it found a missing hydrogen bomb in the Mediterranean Sea. Its next claim to fame was a bit less positive. Alvin sank in 1968 while being transported by the Navy tender ship Lulu. The vessel wasn’t recovered from the sea floor until June 1969, at which point it was towed back to Woods Hole, MA for repair.
In 1973, Alvin‘s steel hull was replaced with a titanium pressure hull. This upgrade would prove instrumental for some of the vehicle’s most important discoveries. In 1979, Alvin discovered what are now known as “black smokers” — hydrothermal vents rich in sulfide-bearing minerals. Black smoker vent fields tend to teem with life, particularly life that survives thanks to chemosynthesis as opposed to photosynthesis. This discovery demonstrated that complex life could exist in the absence of sunlight, especially on distant planets far from the Sun.
The idea that there could be life under Europa’s icecap or inside Enceladus derives from discoveries Alvin made here on Planet Earth. Alvin also carried Dr. Robert Ballard to the first manned exploration of the Titanic wreck since the ship sank on April 15, 1912 and explored the wreck of the USS Scorpion, both in 1986. And 100 years after the Titanic sank, James Cameron became the first solo explorer to reach the Challenger Deep in a submersible named—what else?—the Deepsea Challenger.
Fun fact: Gene Roddenberry named Star Trek captain Jean-Luc Picard after the brothers Auguste and Jean, to recognize their pioneering work in both deep-sea oceanography and high-altitude exploration.
The story of Alvin, Trieste, and innumerable other surface ships and deep-sea exploratory vehicles isn’t just a story of technological innovation. Titanium pressure hulls, sonar, better underwater cameras, and the rise of deep-sea drones have all pushed back the frontier of underwater exploration, but they’ve also helped us understand some of our oldest questions about Earth itself.
There was a time when plate tectonics was a joke of a theory and we had no rich, diverse, and complex examples of chemosynthetic life. Here at ExtremeTech, we often turn our eyes to the heavens and the boundary-pushing wonders captured by Hubble and the James Webb telescope—but some of our most important discoveries about the forces that shaped Earth and gave rise to life itself came not from the skies above our heads but the depths of the ocean.
We hope you’ve enjoyed this deep dive.