Challenger Deep and the Mariana Trench

Podcast Transcript

Located in an arc sweeping to the east and south of the Marina Islands and Guam is the deepest part of the ocean, the Mariana Trench.

Running over 2,500 kilometers or 1,200 miles, the very deepest part of the trench is known as Challenger Deep.

At the very bottom of the sea, there is no light, temperatures are almost freezing, and the pressure is enough to crush almost anything that might make it down there.

It is so inhospitable that the number of people who have ever been there is about the number who have visited the moon.

Learn more about the Mariana Trench and Challenger Deep on this episode of Everything Everywhere Daily.

I’ve done previous episodes on some of the most extreme places on Earth. None, however, comes close to being as extreme as the Mariana Trench and Challenger Deep.

The Mariana Trench is an approximately 2,500-kilometer or 1,200-mile-long crescent on the floor of the western Pacific, south and east of Guam and the Mariana Islands.

Its lowest known point is Challenger Deep, a set of three adjacent sub-basins at the southern end of the trench. Recent bathymetry and submersible surveys show western, central, and eastern “pools,” each more than 10,850 meters deep, with the deepest soundings in the western and eastern pools.

Getting an exact measurement of the deepest point is actually quite tricky.

High-resolution mapping in 2020 refined that picture somewhat.

The problem is that even the Challenger Deep section is a rather large area, so knowing you’ve found “the” deepest spot, rather than a deep spot, is difficult.

The most recent paper from the 2020 measurements puts the deepest point at 10,935 meters with an error of ±6 meters at a 95% confidence interval.

In American units, that converts to 35,876 feet.

To put that into perspective, it is further below sea level than the top of Mount Everest is above sea level, by about 6,000 feet or 2,000 meters.

So, how did such an incredibly deep trench get created on the ocean floor?

The trench formed through a geological process called subduction, where two tectonic plates converge. Specifically, the Pacific Plate is being pushed beneath the smaller Mariana Plate, which is part of the Philippine Sea Plate, at a rate of about 2-3 inches per year.

As the denser oceanic Pacific Plate descends into the Earth’s mantle, it creates this deep depression in the ocean floor. This subduction zone is also responsible for the volcanic Mariana Islands arc that runs parallel to the trench.

This process has been ongoing for millions of years. The Mariana Trench began forming approximately 50-60 million years ago during the Eocene epoch.

There are multiple subduction zones around the world, especially in the Pacific in the Ring of Fire. This is the deepest of them.

The deepest part, Challenger Deep, is named after HMS Challenger, a British Royal Navy survey ship that conducted the first systematic study of the world’s oceans from 1872 to 1876.

In 1875, the crew used weighted sounding lines and discovered an exceptionally deep area in the trench, which they measured at about 8,184 meters or 26,850 feet, though this was significantly underestimated with their primitive equipment.

This deep point was later named Challenger Deep in honor of the ship that made the first measurement.

The next attempt to measure Challenger Deep didn’t take place until 76 years later, in 1951, when the HMS Challenger II, named after the original, returned to the area and used echo sounding to measure a depth of 10,863 meters or 35,640 feet, coming much closer to the currently accepted depth.

In August of 1957, the Soviet vessel Vityaz measured a depth of 11,034 meters or 36,201 feet, which remained the accepted depth for many years, though modern measurements suggest this was slightly overestimated.

Of course, once the deepest point on Earth had been measured, the next logical step was for someone to visit.

Here, I should note that many people have commented that more people have been to space than have visited the bottom of the sea. They often state this in a way that implies it is the opposite of how it should be. Somehow, because Challenger Deep is on Earth, it should be easier to go there than to go into space.

In many respects, going into space is easier. While a sufficiently wealthy person could probably more easily mount a mission to go to the bottom of Challenger Deep than into space, from a pressure standpoint, being in space is much easier.

Once pressure gets down to zero, that’s it. However, pressure can theoretically keep increasing until you reach a black hole.

The most famous early exploration occurred on January 23, 1960, when Swiss engineer Jacques Piccard and U.S. Navy Lieutenant Don Walsh descended to the bottom of Challenger Deep in the bathyscaphe Trieste.

The dive was conducted under the auspices of the U.S. Navy, which had purchased the Trieste from Auguste Piccard, Jacques’ father, and modified it for extreme depths.

The Trieste was not a typical submarine. It was a bathyscaphe, which is a free-diving deep-sea craft designed for vertical descent, not horizontal travel.

The crew sat inside a 7-foot-diameter steel sphere at the bottom of the craft capable of withstanding the immense pressure of the deep. The walls of the sphere were 12.7 cm or 5 inches thick.

There was barely enough for two men and equipment. The pressure inside the sphere was kept at 1 atmosphere, so no decompression was needed after the dive.

Above the sphere was a 15-meter-long float filled with gasoline. Gasoline was used because it is less dense than water and incompressible, making it ideal for buoyancy even under high pressure.

The craft carried iron pellets as ballast. These were released magnetically at the end of the dive to allow the Trieste to rise.

It took 4 hours and 48 minutes to go from the surface to the sea floor, sinking at a rate of a little under 1 meter or 3 feet per second.

They spent about 20 minutes at the bottom before releasing their ballast and rising to the top. The ascent took another 3 hours and 15 minutes.

Just how strong is the pressure at Challenger Deep? It is approximately 1,100 atmospheres or about 16,000 pounds per square inch.

It would be the equivalent of the weight of the Empire State Building being put on an area the size of your foot.

An average naval submarine is rated to dive to only about 3% of the depth of Challenger Deep.

No one else would make the trip to the bottom of Challenger Deep for another 52 years. The next person to do it was director James Cameron, who descended in the Deepsea Challenger in 2012. He spent 3 hours at the bottom and recorded what he saw in high definition video.

As of the recording of this episode, 27 people have been to Challenger Deep, whereas only 24 people have ever landed or been in orbit around the Moon.

Of special note is Victor Vescovo. He made the 3rd-ever dive in 2019 and has completed a total of 15 Challenger Deep descents.

Two astronauts have been to Challenger Deep: Kathryn Sullivan, who went in 2020, and Richard Garriott, the creator of the Ultima series of games, who went in 2021.

So, what is it like down there?

Challenger Deep lies in what is known as the hadal zone. The hadal zone gets its name from the word Hades.

For starters, there is a whole lot of nothing. People who have been there have described it as being like an enormous salt flat. It is mostly sediment with few recognizable features.

It is also totally dark. There is no light beyond about 300 meters. Challenger Deep is basically like being inside a cave in terms of light.

The temperature of the water is about 34 degrees Fahrenheit or about 1 degree Celsius, just above freezing.

This extremely cold, dense water originally sank near the poles, mainly in the Southern Ocean around Antarctica and the North Atlantic, where surface water becomes cold and salty enough to sink to the ocean floor.

Once that water sinks, it spreads slowly along the bottom of the world’s oceans, following gradients in temperature, salinity, and seafloor topography. It creeps along like an invisible current system thousands of meters below the surface.

At the depth of Challenger Deep, circulation is exceptionally sluggish. Water there is the oldest in the world ocean because it is the end point of the conveyor belt. Because it is isolated from most bottom currents, water in Challenger Deep may be 2,000 years old or more before being replaced.

Vertical mixing of water is weak but not nonexistent. Occasional deep internal waves, earthquake-induced turbulence, and density-driven microcurrents allow some exchange of water and dissolved gases between the trench and the adjacent abyssal plain.

Water also acts differently with this much pressure. Under normal conditions, water doesn’t compress. This is why hydraulics work.

However, at Challenger Deep, water does slightly compress. At the surface, seawater density averages about 1.027 g/cm³. At the bottom, it is 1.071 g/cm³.

That’s about a 4–5% increase in density compared to surface seawater, which is actually enormous for water.

As a result, the speed of sound is considerably faster at around 1500–1550 m/s,

With these extreme conditions, many of you are probably wondering, Is there life this far down?

The answer is….yes, though it’s unlike anything we encounter near the surface.

The problem is that there is no light and very little dissolved oxygen in the water.

There are a fair number of species of microbes at this level that have been discovered in multiple missions to Challenger Deep. Many of these are piezophiles, which means pressure-loving. These microbes metabolize carbon, nitrogen, sulfur, and even methane, forming the base of a hadal food web.

Among multicellular creatures, there are Amphipods, small shrimp-like crustaceans, that have been observed. They survive by scavenging organic detritus that sinks from above.

Nematodes and sea cucumbers have also been observed in sediment samples.

One thing that has never been observed is fish or other vertebrates. The deepest known fish, the hadal snailfish, lives on the trench slopes up to about 8,200 meters, but not deeper. It is likely because cellular membranes and enzymes can’t function properly beyond about 8,400 to 8,600 meters.

There is one other thing that has been found at the deepest point on Earth……garbage.

When explorers first reached Challenger Deep in 1960, they reported seeing only pale sediments and a solitary creature. At the time, it was considered a pristine environment untouched by human activity.

In 2019, during the Five Deeps Expedition, Victor Vescovo filmed what appeared to be a plastic bag and several candy or food wrappers lying on the seafloor, at nearly 10,900 meters.

Later dives found other fragments of plastic and fabric mixed into the sediment. Similar findings were reported by Chinese expeditions in 2020 and 2021, which retrieved samples containing microplastic fibers from even the most remote parts of the trench.

Laboratory analyses of amphipods collected from Challenger Deep revealed that their bodies contain traces of persistent organic pollutants, some of which had been banned decades ago.

Because water at this level moves so slowly, whatever makes it this far down will remain for potentially thousands of years.

Challenger Deep is the most extreme environment on planet Earth. It is extremely difficult to get to, and once there, you can’t stay very long.

Yet, despite being so inhospitable, life manages to find a way.

Life…..and garbage.