Hades is the classical Greek word for the underworld (Hell). The hadal zone in the sea is defined as depths between 20,000 feet and the maximum depth of seven miles.
Deep ocean trenches make up the World Ocean's "hadal" zone. With depths greater than 20,000 feet, this area of the world is larger than the U.S. but largely unexplored. Image by TASA Graphic Arts.
The seafloor of the hadal zone is only two percent of the total World Ocean area. This region is mostly deep ocean trenches (33) and troughs (13). While two percent seems small, it is equivalent to the area of the continental United States, Alaska, and about half of Mexico. This zone is the least known part of our planet.
The “Ocean Deeps” were first discovered by the British Challenger Expedition (1872-1876). The word “trench” was not adopted until after World War I. The Danish Galathea Expedition (1950-1952) did physical sampling and depth measurements to 33,430 feet in the Philippine Trench. Also in the 1950s, sufficient seafloor mapping was completed to determine that Challenger Deep, in the Mariana Trench, was the ocean’s deepest place at about 36,000 feet—the pressure there is 16,000 psi.
In addition to the Mariana Trench, there are six other trench systems with depths greater than 30,000 feet: Philippine, Tonga, Japan, Izu-Bonin, Kurile-Kamchatka, and Kermadec. Most are in the Pacific.
Early explorations by surface ships were complicated and difficult. Depth-measuring equipment and associated mapping systems were primitive and rare. Greatest depths were sometimes mapped using explosive charges where the time between the initial detonation and the return echo was converted to the physical depth. Physical sampling and data gathering was accomplished by lowering devices on long, tapered wires.
A major problem was that samples and data collection were more opportunistic than systematic. Animals captured by spot sampling did not give an idea of their populations, density, or interactions within their benthic biome. Nevertheless, expeditions to date have found nearly 400 species living in 21 trenches.
In the 1960s, development of manned and unmanned submersibles provided a means of in situ explorations. For the first time, the trained minds and eyes could go deep inside the sea. It began in 1960 when the U.S. Navy’s bathyscaph Trieste dove to the seafloor at Challenger Deep. Next were two unmanned vehicles, Japan’s Kaiko in 1996 and 2009 and the U.S. vehicle Nereus in 2007. James Cameron piloted his Deepsea Challenger submersible to the seafloor at Challenger Deep in 2012.
Cameron’s expedition also employed a “lander” submersible. Untethered landers are dropped into the sea, sufficiently weighted to sink to the seafloor. These simple vehicles are productive, reliable, and low cost. They are equipped with bait to attract animals, cameras to photograph them, and various instruments to record data. Lander missions can last hours to weeks. For recovery, they are fitted with ballast weights that are dropped when pinged by acoustic signal or when commanded by a timer. On the surface, signaling devices ensure recovery.
Research work in the greatest ocean depths has increased significantly in the last half decade. Improved mapping systems provide detailed topographic seafloor charts while new generations of unmanned submersibles provide imagery, data, and physical samples. Programmed untethered autonomous unmanned vehicles are now able to systematically survey large seafloor areas. In the future, virtually all in situ research in the hadal zone will use unmanned submersibles. Compared to manned systems, they are reliable, low cost, safe, and productive.
Given the right equipment, more and more oceanographers are now happily going “to Hell.” But for the near term, the hadal depths largely remain a mare incognita.