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James Cameron made headlines in March by successfully diving 6.8 miles (11 kilometers) to the deepest part of the Mariana Trench in a one-person submersible[2] called the Deepsea Challenger. Best known as a Hollywood director, Cameron is one of just three people to dive to Earth’s deepest point, and the only one to be able to stay long enough to look around. Though Cameron and his team are hoping to discover more about the biology of the Mariana depths, geologists already know a lot about how the Mariana Trench formed—and why it’s Earth’s deepest spot. Three Major Factors that Made Earth’s Deepest Spot The Mariana Trench isn’t really the deep, narrow furrow that the word “trench” implies. Rather, the abyss marks the location of a subduction zone.[3] Subduction zones occur where one part of the seabed—in this case the Pacific plate—dives beneath another, the Philippine plate. Tectonic forces eventually make the Pacific plate dive into the Earth’s interior almost vertically, but at seabed level the plate dips at a relatively gentle angle.[4] Robert Stern, a geophysicist at the University of Texas, said, “One reason the Mariana Trench is so deep is because the western Pacific is home to some of the oldest seafloor in the world—about 180 million years old. Seafloor is formed as lava at mid-ocean ridges.[5] When it’s fresh, lava is comparatively warm and buoyant, riding high on the underlying mantle.[6] But as lava ages and spreads away from its source, it slowly cools and becomes increasingly dense, causing it to settle ever lower—as is the case with the Mariana Trench.” | 
