Hadal zone

Key Takeaways

The hadal zone occupies the deepest ocean trenches, extending from 6,000 to 11,000 meters (about 36,000 feet) down — a combined area roughly the size of Australia.
Most trenches form at subduction zones, where one tectonic plate slides beneath another — movement that can trigger the earthquakes and tsunamis that reach coastal communities.
Life here endures pressures more than 1,000 times those at the surface; WHOI and NASA’s Jet Propulsion Laboratory built the autonomous Orpheus vehicles to reach and study it.

The hadal zone is the ocean’s deepest realm, plunging from 6,000 to 11,000 meters inside trenches carved where tectonic plates collide. Combined, these trenches cover an area the size of Australia and shelter life found nowhere else — which WHOI and NASA reach with the pressure-proof Orpheus vehicles.

What is the hadal zone?

In most areas, the ocean floor lies 4,000 to 6,000 meters (13,000 to 20,000 feet) below the surface, but deep ocean trenches can extend this depth to 11,000 meters (36,000 feet). The region extending from 6,000 to 11,000 meters is called the hadal, or hadalpelagic, zone after Hades, the Greek god of the underworld. This zone occurs only in trenches; combined across all oceans, they make up an area about the size of Australia. The hadal zone is characterized by extreme depth and pressure, temperatures that hover just above freezing, and complete darkness—at least in terms of light from the sun. Bioluminescence allows organisms in the ocean depths to communicate. Walls of trenches are steep and descend through a gradient of depths and pressures, creating a variety of habitats along the slope.

Why is it important?

Trenches most often occur where one tectonic plate is subducted under another, creating long, narrow submarine canyons. Movement of one plate over another can cause earthquakes and tsumanis that affect people on land, and understanding tectonic motion is essential for warning people who might be affected. More often, however, Earth's hadal seafloor is found mid-plate in regions that have never been explored.

The steep walls leading to hadal regions are home to life forms found nowhere else on Earth. Studying these organisms can help us better understand adaptation to extreme environments. Some of these organisms rely on chemical energy from hydrothermal seeps along the ocean floor. Others depend on carbon-based matter that falls from upper layers of the ocean. Accumulation of carbon along the base of the trenches may play a key role in the carbon cycle and climate regulation. Exploring the hadal zone will advance knowledge that can be used when exploring oceans beyond Earth, such as those on the moons of Jupiter and Saturn.

A map of hadal regions in the Pacific shows that most of the ocean’s deep seafloor does not lie in the long subducting trenches that border tectonic plates, but mid-basin, where very little exploration has occurred. Illustration by WHOI Creative, ©Woods Hole Oceanographic Institution.

What are ocean scientists doing to better understand the hadal zone?

Researchers are actively working to investigate trenches and the waters within them. To do so, WHOI is collaborating with NASA’s Jet Propulsion Laboratory to develop technology that allows exploration of the hadal zone. This effort is not without challenges, given the remote location and extreme topography and pressures found in ocean trenches.

Together, WHOI and NASA have developed Orpheus autonomous underwater vehicles (AUVs). These small AUVs can withstand pressure greater than 1,000 times that at the ocean’s surface (as occurs at the deepest points), and they can navigate narrow, rocky sections of trenches. Orpheus AUVs can work independently or in a “swarm” to explore, map, and analyze the water, seafloor, and organisms in the hadal zone. One of their primary benefits is their ability to reconfigure their mission on the fly if they encounter unexpected issues.

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