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.”

Two other factors conspire to make the 1,580-mile-long (2,550-kilometer-long) Mariana Trench staggeringly deep.[7] For one, the trench lies far from any major landmass, which means it’s remote from the mouths of muddy rivers. “Many other deep trenches are more filled with sediment,” Chris Goldfinger, professor of Oregon State University, explained. “This one isn’t.” In addition, nearby fault lines cut the Pacific plate into a narrow tongue at the site of the trench, allowing the plate to bend more steeply downward than at other subduction zones.[8]

Cameron Dive a “Man on the Moon” Feat

Despite Cameron’s record dive, it’s impossible to know what really happens in the subduction zone, since most of the action occurs up to 420 miles (700 kilometers) below Earth’s surface. “That’s the iceberg,” Stern said. “Cameron wasn’t even at the tip of it—11 kilometers out of 700. The trench is the interface between the limits of human experience and the reality humans can’t experience.”

Even so, with subsequent dives, there’s the prospect of retrieving rock samples and looking for life deeper than it’s ever been found before. Cameron commented, “It’s really the sense of isolation, more than anything, realizing how tiny you are down in this big vast black unknown and unexplored place... I see this as the beginning. It’s not a one-time deal and then moving on. This is the beginning of opening up this new frontier.” Stern agreed, calling Cameron’s trip “kind of a man-on-the-moon thing.”