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The Mariana Trench is 7 Miles Deep—What's Down There?

The Mariana Trench in the Pacific Ocean is so deep your bones would literally dissolve. What's down there in its black, crushing depths?

By
Sabrina Stierwalt, PhD
Episode #351
mariana trench
The Quick And Dirty

The Mariana Trench is a crescent-shaped dent in the floor of the Pacific Ocean, extending over 1500 miles long with an average width around 43 miles and a depth of almost 7 miles (or just under 36,201 feet). For comparison, most ocean life lives above a depth of 660 feet. Nuclear submarines hover around 850 feet below the surface as they travel through the ocean waters. Whales aren’t usually seen below about 8,200 feet. The site of the sunken Titanic, can be found at 12,467 feet. Bacteria and small invertebrates are able to survive in the deepest spot.

Somewhere between Hawaii and the Philippines near the small island of Guam, far below the surface of the water, sits the Mariana Trench, the deepest spot in the ocean. What's down there?

How deep is the Mariana Trench?

The Trench sits like a crescent-shaped dent in the floor of the Pacific Ocean, extending over 1500 miles long with an average width around 43 miles and a depth of almost 7 miles (or just under 36,201 feet). At that depth, the weight of all that water above makes the pressure in the Trench around 1000 times higher than it would be in, say, Miami or New York. Floor vents release bubbles of liquid sulfur and carbon dioxide. Temperatures are just above freezing, and everything is drowning in darkness. 

For comparison, most ocean life lives above a depth of 660 feet. Nuclear submarines hover around 850 feet below the surface as they travel through the ocean waters. Whales aren’t usually seen below about 8,200 feet. The site of Jack and Rose’s true (albeit fictional) love, the sunken Titanic, can be found at 12,467 feet.

According to National Geographic, if you were to put Mount Everest at the bottom of the Mariana Trench, its peak would still sit around 7,000 feet below sea level

Toward the southern end of the Mariana Trench lies the Challenger Deep. It sits 36,070 feet below sea level, making it the point most distant from the water's surface and the deepest part of the Trench.

If you were to put Mount Everest at the bottom of the Mariana Trench, its peak would still sit around 7,000 feet below sea level.

While the number of people that have climbed to the top of Mount Everest, the Earth’s highest point, holds somewhere in the thousands, only 3 divers have ever explored the Challenger Deep. The first expedition happened in 1960 when Jacques Piccard and Navy Lt. Don Walsh reached the Challenger Deep in a U.S. Navy submersible. They were only able to spend 20 minutes there due to the extreme pressures, and their arrival stirred up too much dust from the seafloor for them to take any pictures. 

The next visitor didn’t arrive until over 50 years later in 2012, when filmmaker and science fiction aficionado James Cameron solo dived to the Challenger Deep in a submarine he designed himself. Cameron was able to spend three hours there. And, of course, he captured video and took many photos—he is a Hollywood filmmaker, after all.

The extreme pressures took a toll on his equipment, though. Batteries drained, sonar died, and some of his vessel's thrusters to malfunctioned, making it hard to maneuver. 

Why do we explore the trench?

Our limited knowledge of the Mariana Trench means we have explored further into outer space than we have into the depths of the ocean. And just as our space exploration efforts are driven by the question "Is there life out there?" so has been our desire to explore the trench.

Your bones would literally dissolve at that depth.

Before 1960, marine biologists were skeptical that anything could live under such extreme conditions. Some still think that vertebrates would not survive at such high pressures because calcium can only exist in solution. That means your bones would literally dissolve at that depth.

But even during their short visit, Piccard and Walsh reported signs of life, including what they said looked like a flatfish. (Although some scientists later argued that it must have been more of a sea cucumber.) On his expedition, James Cameron described the floor of Challenger Deep as “very lunar,” as in fairly barren when it came to life. But his investigations revealed new species of bacteria and some small invertebrates that were all able to make a life for themselves even in the deepest spot in the Trench. 

At slightly shallower depths (think very, very deep instead of very, very, very deep), expeditions have found a variety of ocean life that make the rest of the Trench their home. In 2015, the Okeanos Explorer, led by the National Oceanic and Atmospheric Administration (NOAA), found a tiny, nearly translucent octopus which appears to lack the color-changing abilities of other octopi, a “sponge the size of a minivan”, and a ghostly white fish that has been described as a mini version of Falcor, the big white dragon from the Neverending story.

Understanding how life can survive under such extreme conditions can help us understand how life may have originated on Earth when our planet was hotter and more volatile than it is today. Researchers also hope the microorganisms they discover at such extreme depths may have uses in biomedicine. Recently, scientists have noted that some of these unique bacteria in the Trench appear to be oil-eating.

How did the Mariana Trench form?

The Mariana Trench isn’t alone: there is a whole network of deep trenches that carve their way along the bottom of the ocean. They are a result of the collision of two tectonic plates. When two of the slabs of rock that make up the surface of the Earth move and crash into each other, one slab can be forced underneath the other, creating a trench. 

Earlier this year came the third claim of discovering a plastic bag in the Trench, this time from a diver exploring as far down as 35,849 feet.

In 2009, then-President George W. Bush set up the Marianas Trench Marine National Monument, which created a U.S. protected zone over most of the Trench. The U.S. has jurisdiction over most of the trench because it is closest to Guam, a US territory. Research permits are now required to visit, but that hasn’t stopped the slow creep of plastic waste that has now infiltrated even the deepest depths of the ocean. Earlier this year came the third claim of discovering a plastic bag in the Trench, this time from a diver exploring as far down as 35,849 feet. Sigh.

About the Author

Sabrina Stierwalt, PhD

Dr. Sabrina Stierwalt is an extragalactic astrophysicist at the California Institute of Technology and Adjunct Faculty at the University of Virginia.

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