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Where is Challenger Deep?

Detail from Bathymetrical Chart of the Oceans Showing the Deeps (1912)

James Cameron’s recent voyage to the bottom of the sea made headlines worldwide. As well it should have–Cameron is the first person to venture alone to the deepest known part of the world’s oceans. His “vertical torpedo” submersible dove nearly 11,000 meters to a place he described as “lunar” and “desolate”–a place often called Challenger Deep.

But where–and what, exactly–is that?

Challenger Deep takes its name from the H.M.S. Challenger, a 19th century Royal Navy ship that circumnavigated the globe to study the world’s oceans. On March 23, 1875,  some 140 miles southwest of Guam at the far western edge of the Pacific Ocean, Challenger crew did something they would eventually do more than 500 times during their three-and-a-half-year-long expedition–drop a hemp rope weighted with iron sinkers into the water for the purpose of measuring the sea floor’s depth. The outcome: 4,475 fathoms (8,184 m). The sounding wasn’t a record, but it was the first taken in the vicinity of what we now know is home to the deepest spot on Earth, the Mariana Trench.

The name “Challenger Deep” is often used synonymously with Earth’s deepest point, but Challenger Deep isn’t a sounding or a single spot on the ocean floor. It’s a “deep.” More on this in a moment.

Early bathymetric maps were often wrong about the precise nature of sea floor topography, but they nonetheless portrayed deeps as submarine regions rather than discrete points or soundings.  For example, when John Murray, oceanographer and Challenger Expedition member, published “Bathymetrical Chart of the Oceans Showing the Deeps” in 1899, he classified deeps as parts of the ocean deeper than 3000 fathoms (5,486 m). Accordingly, Challenger Deep shows up as a dark, bean-shaped region, south and a little west of Guam, about the same size as Massachusetts, Connecticut, and Rhode Island combined.

Murray published an updated map in 1912 [see above] that depicts a much-enlarged Challenger Deep–one that pretty closely resembles the crescent-shaped Mariana Trench–and one whose area Murray estimated at 129,000 square miles. That’s nearly four times as large as Lake Superior–hardly a point on a map.

But as our knowledge of the Earth’s underwater landscape has grown over the last century, so has our vocabulary for it. Ocean deeps are now outnumbered on modern maps by troughs, canyons, and trenches. These “new” underwater features are usually labeled the way other linear and areal topographic features are: with labels that accent the sizes and shapes of the features they describe.

Detail from a 2012 National Geographic map of the world.

And the deeps? They’ve been reduced to points on the map. Which, in turn, suggests that they’re no longer considered areas. Except that they are: both the U.S. Board on Geographic Names Advisory Committee on Undersea Features and other international naming authorities continue to define deeps as “localized deep area(s) within the confines of a larger feature, such as a trough, basin or trench.”

To be fair, cartographers depict and label ocean deeps the same way they depict and label individual mountains–with a name and a point used to indicate distance from sea level.  But cartographers and readers know that mountains are more than their summits. The same is true for deeps, it turns out. No matter how they’re labeled.

Michael Fry
Senior Map Librarian
National Geographic Society