Oceanic trenches represent the most profound and least explored regions on our planet, forming long, narrow depressions across the ocean floor. These immense geological features are vast, crescent-shaped scars in the Earth’s crust, marking the deepest parts of the global ocean basins. Studying these extreme environments provides scientists with unique data on the planet’s internal processes and the limits of life. They are places of crushing pressure, perpetual darkness, and near-freezing temperatures, making exploration a significant technological challenge.
The Planet’s Second Deepest Point
The second deepest point on Earth is the Horizon Deep, located within the massive Tonga Trench in the southwestern Pacific Ocean. The Tonga Trench stretches approximately 2,500 kilometers and is characterized by a high rate of tectonic movement. The Horizon Deep reaches a measured depth of approximately 10,823 meters (35,509 feet) below the sea surface. It is the deepest point in the Southern Hemisphere.
The area was first identified in 1952 by the research vessel Horizon, which gave the deep its name. Explorer Victor Vescovo completed the first manned solo dive to the bottom of the Horizon Deep in 2019. This expedition provided highly detailed sonar mapping, definitively confirming its depth and establishing it as the world’s second deepest location.
Understanding the Mariana Trench and Challenger Deep
To understand the scale of the Horizon Deep, it must be compared to the world’s deepest known point, the Challenger Deep, located in the Mariana Trench. The Mariana Trench is in the western Pacific Ocean, southwest of Guam. The Challenger Deep is the lowest point within this trench, reaching an estimated maximum depth of 10,935 meters (35,876 feet). This measurement was obtained by modern deep-diving submersibles.
The depth difference between the first and second deepest points is small, with the Horizon Deep being only about 105 meters shallower than the Challenger Deep. If Mount Everest were placed into the Challenger Deep, its peak would still be covered by more than a mile of water. The extreme pressure at the bottom of the Challenger Deep is over 1,000 times the atmospheric pressure experienced at sea level.
How Subduction Zones Create Extreme Depths
The existence of the world’s deepest trenches, including the Tonga Trench and the Mariana Trench, is a direct result of a geological process called subduction. This process occurs at convergent plate boundaries, where two of the Earth’s massive tectonic plates collide. One tectonic plate, typically older and denser, is forced to slide beneath a lighter plate into the Earth’s mantle.
As the descending plate bends sharply downward, it creates a deep, narrow depression in the seafloor at the point of contact. The Tonga Trench, for example, is formed where the Pacific Plate is actively subducting beneath the Tonga Plate. The Pacific Plate is currently subducting at one of the fastest rates on Earth, which contributes to the immense depth of the resulting trench.