Many of us may wonder about the extremes of our planet — what is the highest mountain, what are the coldest temperatures ever recorded, or what is the deepest point in the ocean?

When it comes to that last point, oceanographers have long known that the deepest part of the world ocean occurs in the Mariana Trench, a deep valley in the ocean floor that lies about 124 miles east of the Mariana Islands in the western Pacific Ocean. And for many years, the consensus view among those scientists was that the maximum depth of the Trench — nearly seven miles, which is a mile deeper than Mt. Everest is tall — occurs at the southern end of the Trench in a depression known as the Challenger Deep.

Unfortunately, scientists have not been able to agree on precisely how deep the Challenger Deep is, in spite of multiple measurements over the years by deep-diving submersibles, sonar bathymetry and remotely-operated underwater vehicles.

But that all changed in December 2014 during the voyage of the Schmidt Ocean Institute’s Research Vessel Falkor to the Mariana Trench.

Exploring the Challenger Deep

According to Oceanography magazine, the expedition planned to gather new data about the waters above the Challenger Deep using two autonomous, deep-diving instruments known as Deep Sound Mk II and Deep Sound Mk III. The instruments would help the scientists complete several tasks:

• Create conductivity, temperature, and depth profiles of the water column.
• Collect and create video recordings of hadal amphipods.
• Record the ambient sounds of the ocean within the 5 Hz to 30 kHz band of frequencies.

The two instruments were designed to descend under gravity until a preprogrammed condition was met, such as a pre-set depth or an elapsed time, then drop an iron weight and ascend to the surface under their own buoyancy.

Feeling the Pressure

The two instruments were deployed on December 17, 2014. According to National Geographic, as they descended, David Barclay — a member of the expedition and an associate professor of oceanography at Dalhousie University in Nova Scotia — was listening through headphones to an underwater microphone mounted on the ship’s hull when he heard a faint pop, something he wasn’t expecting.

He later figured out that the pop resulted from the implosion of the 15-inch-wide spherical glass housing that encased the Mk III’s electronics. The instrument, which was at a depth of more than 8,000 feet at the time, was a total loss, but serendipitously, the chain of events that ensued led to one of the most precise measurements to date of the Challenger Deep.

Finding Success in Serendipity

When the Mk III imploded, it created a very energetic shock wave that was recorded by the surviving Mk II instrument. The shock wave reflected multiple times between the ocean surface and the seafloor of the Challenger Deep. By noting the arrival times of these multi-path pulses and knowing the speed of sound in the water column at the time of the implosion, the Falkor scientists were able to calculate with great precision the depth of the Challenger Deep — an astounding 10,983 meters +/- 6 meters, or about 36,033 feet.

SurferToday notes that prior to the Falkor expedition, oceanographers believed the Challenger Deep was between 35,755 and 35,853 feet deep.

How Do Scientists Measure the Depth of the Ocean?

According to the Woods Hole Oceanographic Institute, efforts to measure the deepest point in the ocean date back to 1872 to the voyage of the HMS Challenger, the world’s first oceanographic expedition. Perhaps Challenger’s most significant finding was the discovery of the Mariana Trench. But how do scientists measure the depth of the ocean without modern technology? As Oceanography magazine explains, back then, they used traditional depth-sounding methods — spooling out a rope marked in six-foot intervals until the weight at the end of the rope hit bottom. The Challenger’s crew recorded a depth of 8,140 meters.

Since then, the technology to measure ocean depth has evolved dramatically.

One of the most common instruments used to measure ocean depth is called an echo sounder, which comprises a transmitter and a receiver. The transmitter sends a sound pulse of about 12 kHz straight down into the water. The pulse reflects off the ocean floor and returns to the surface where the echo sounder’s receiver detects it. Knowing the velocity of sound in that column of water and the time for the signal to make its round trip, scientists can calculate the depth of the ocean floor at that position.

Measuring Uncertainty

And therein lies the uncertainty in echo sounder measurements.

“The speed of sound in the water column is not a constant value but actually varies with salinity, temperature and pressure,” explains Barclay. “To calculate ocean depth, we need to know the speed of sound along the path of sound waves emitted by the echo sounder.”

He explains that, to overcome this problem, oceanographers typically create a sound speed profile of the water where they’re operating. But even then, air in the water column can change the speed of sound in that water, which introduces depth measurement errors on the order of meters if not tens of meters.

Today, organizations such as the National Oceanic and Atmospheric Association (NOAA) use multibeam echo sounders to map the ocean floor. In 1951, the HMS Challenger II used an echo sounder to record a depth of 10,863 meters in the part of that would later be known as the Mariana Trench.

Going Deep With Submersibles

Oceanographers also use instruments attached to submersible vehicles, both manned and remotely operated, to determine the deepest point in the ocean.

In January 1960, oceanographer Jacques Piccard and U.S. Navy Lt. Done Walsh descended into the Challenger Deep in the bathyscaphe Trieste. The underwater vehicle’s instruments recorded a depth of 10,916 meters.

In 2012, filmmaker James Cameron made the second crewed dive to the bottom of Challenger Deep in his submersible DeepSea Challenger, which recorded a depth of 10,912 meters.

As Barclay sees it, taking an instrument to the bottom of the Challenger Deep is probably still the best way to get the most accurate measurement. But even this “up-close and personal” approach is not without imprecision.

“If your vehicle moves too quickly and stirs up the water too much as its instruments measure water density, that can throw your measurements off a bit, which again introduces uncertainty in your final measurements,” he observes.

The Bottom Line

Barclay proposes that the real question is whether small uncertainties in determining the deepest point of the ocean really matter. He acknowledges what he calls an “ongoing contest among billionaires to find the deepest part of the Mariana Trench,” but “from a U.S. Navy or NOAA standpoint, I think most of these tolerances are relatively acceptable.”

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