Which of these places is the deepest geographical location on Earth?

It was once thought that the ocean floor was a vast, flat, featureless plain. In the 1840s, explorers began to measure the depth of the ocean and came to the realization that there was as much variety in the contours of the ocean floor as there was on dry land. It is at the bottom of the Pacific Ocean that we find the deepest known point on Earth, the Challenger Deep, a small, slot-shaped depression in the southern end of the Mariana Trench, the deepest trench in the world.

Named for the ship that discovered the chasm, the Challenger Deep was first detected in 1875 by the crew of the HMS Challenger, an British ship on a four-year marine science expedition. Between 1872 and 1876, the Challenger circumnavigated the globe, traveling almost 69,000 miles, making it the longest oceanographic survey ever conducted. The crew took 492 deep sea soundings (a water depth measurement) and 133 bottom dredges, including their deepest sounding of 4,475 fathoms, or about 26,850 feet, in what became known as the Challenger Deep. According to the written report, “such deep water not having been expected,” they took two additional soundings to verify the measurement at the location, 189 miles southwest of Guam (11° 22’ 24” N, 142° 35’ 30” E).

Fifty volumes of scientific reports followed the conclusion of the expedition, filled with maps, drawings, and photographs of all that had been discovered on the voyage. In 1877, German cartographer August Petermann produced Tiefenkarte des Grossen Ozeans (Depth Map of the Great Ocean), highlighting the Pacific Ocean portion of three recent oceanic expeditions (the Tuscarora, the Challenger, and the Gazelle). This map, seen below, was the first to name the discovered point Challenger Deep (or “Challenger Tief”).

Tiefenkarte des Grossen Ozeans by August Petermann, 1877. Geography and Map Division, Library of Congress.

Detail of Challenger Deep from Tiefenkarte des Grossen Ozeans by August Petermann, 1877. Geography and Map Division, Library of Congress.

It is interesting to note in the map above that at this point, Challenger Deep was not yet confirmed to be the lowest known point on Earth. In 1874, the USS Tuscarora sounded a deeper point, at 27,390 feet, in what is now called the Japan Trench. The soundings at neither the Japan Trench nor the Challenger Deep plumbed the true depths of these places. Years later, with improvements in technology, surveyors found that the canyons went much deeper. In 1951, 76 years after its original discovery, the entire Mariana Trench was surveyed by another Royal Navy vessel, also named HMS Challenger, after the original expedition ship. This survey recorded the deepest part of the trench at 5,960 fathoms (35,760 feet) using echo sounding, a more precise method to measure depth. To put that depth in perspective, if Mount Everest was dropped onto the floor of the Challenger Deep, there would still be more than a mile of water between the summit and the ocean surface!

The detail of the map below, produced by the Unites States Hydrographic Office, shows bathymetric contours of a portion of the Mariana Trench, including the Challenger Deep in the lower left corner, several years prior to the 1951 reading.

Bathymetric Chart Korea to New Guinea including the Philippine Sea by the Unites States Hydrographic Office, 1946. Geography and Map Division, Library of Congress.

Though several unmanned vessels have reached the Challenger Deep, only three people have ever landed on the bottom, seven miles under the surface. In January 1960, a bathyscaphe named Trieste took Jacques Piccard and Lieutenant Don Walsh to the very bottom of the ocean floor. They were only able to spend about 20 minutes in the deep, as they became concerned when an outer window cracked from the pressure, causing them to abort the rest of the mission. The only other manned voyage to Challenger Deep occurred in March 2012, when Canadian film director James Cameron made a solo descent in the Deepsea Challenger. Cameron stayed at the bottom for about two and a half hours of his planned six hour trip before mechanical failures also forced him to return to the surface.

Mapping the ocean floor is a difficult and time-consuming process. One of the most famous maps of the ocean floor was created by dedicated oceanographic cartographers Marie Tharp and Bruce Heezen. Below, the manuscript map depicting their work, hand-painted by renowned artist and cartographer Heinrich C. Berann, represents the first systematic attempt to map the entire ocean floor. The shape and contours of the Mariana Trench can be seen in remarkable detail.

Manuscript painting of Heezen-Tharp World ocean floor by Marie Tharp, Bruce Heezen and Heinrich C. Berann, 1977. Geography and Map Division, Library of Congress.

Detail of Mariana Trench from Manuscript painting of Heezen-Tharp World ocean floor by Marie Tharp, Bruce Heezen and Heinrich C. Berann, 1977. Geography and Map Division, Library of Congress.

Though much more is known about the Mariana Trench, the Challenger Deep, and the ocean floor today than in those first years of exploration, there is still so much more to learn. Much of the ocean remains unexplored, leaving an abundance of lingering mysteries in the deep underwater places on Earth.

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