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The deepest parts of our oceans, the region between 6000 and 11��0000��m depth, is known as the hadal zone, and it represents some of the most active and diverse marine habitat on Earth.

Many studies of the hadal zone focus on the conspicuous, gigantic trenches formed by shifting tectonic plates, known as subduction zones. Each of these trenches was thought to represent isolated environments, separated from each other by areas of much shallower water depths.

For decades, researchers studying the hadal zone have had to cope with the technical challenges of working at such great depth. This means understanding of these traditionally difficult to study regions has lagged behind the overlying, shallower water ecosystems. New technologies are changing this imbalance, and there are now more opportunities to explore and study the planet’s most extreme environment; the hadal zone included.

Understanding more about these deep oceans — some of the most remote and inaccessible habitats on Earth — is crucial for the regulation of global climate and for our understanding of biogeochemical cycles. They provide insights into hazards from submarine landslides and large earthquakes, and will provide invaluable baseline data as human exploration for raw materials ventures into ever deeper waters.

In a recently published study, ‘‘, led by Newcastle University in collaboration with ���ճԹ�, scientists attempted to evaluate the true extent of the deepest marine ecosystem by focusing on the North-west Pacific Ocean.

A number of large, subduction trenches are located here, including the deepest point on Earth (Challenger Deep) in the Mariana Trench, a crescent-shaped, 2550 km-long subduction zone that includes the Volcano Trench. At 10 925 m deep, it would immerse Mount Everest with still more water above it and could house 34 Eiffel Towers stacked on top of each other.

Whilst subduction trenches, like those that make up the Pacific ‘Ring of Fire’, are obvious hadal ecosystems, this study explored other geological features such as fracture zones and broad ocean basins. These features, although numerous, are less prominent and are often overlooked.

By evaluating the frequency and distribution of these lesser-known smaller features, the researchers conclude that, in the North-west Pacific, the total hadal area is four times larger than that of trenches alone — roughly equivalent to the size of Alaska, Texas and Montana combined.

In fact, it makes up a staggering 2 793 011 km²; an area considerably larger than the 686 114 km² accounted for by subduction trenches alone.

Having previously thought the Mariana Trench to be one continuous hadal habitat, the scientists discovered it is in fact five isolated areas, with the most northern being what Russian scientists used to call the Volcano Trench.

The study went on to identify that there are no physical partitions either north or south of the Japan Trench to isolate it from the neighbouring Kuril-Kamchatka or Izu-Bonin trenches, and so it can be concluded that it forms one continuous hadal habitat.

The Mariana-Volcano chain of trenches is more complex than expected. This study proves that there are actually four topographic highs that partition the hadal areas of this subduction zone within what has previously been considered by some to be a single feature.

From a habitat perspective, it means there are five physically isolated hadal habitats making up the Mariana-Volcano complex, which means that the genetic flow between hadal populations of species endemic to these extreme depths will be limited.

This means that hadal depths represent a diversity of species, each potentially carrying hidden secrets that will be important for us to discover and learn from.

Heather Stewart, ���ճԹ� Marine Geologist and co-author of the study.

The study also revealed that the Yap Trench actually comprises two separate hadal areas.

However, the Izu-Bonin, Japan and Kuril-Kamchatka trenches lack a biologically significant shallow-water divide. These three trenches seem to represent one continuous hadal corridor.

Aside from advancing geological understanding, scientists are particularly interested in these findings from a biological perspective. They are especially keen to understand whether geographically isolated populations may have reduced levels of gene flow between them, causing them to diverge morphologically. 

By studying the geomorphology, or shape, of the sea floor, we have revealed which trenches form continuous hadal habitats and which are completely isolated from each other. When you include the previously overlooked hadal areas located away from these subduction zones, the total hadal ecosystem is potentially massive.

Heather Stewart.

In the , two main organisms have been used to study hadal endemism: amphipods and fishes.

Amphipods are typically collected consistently in large numbers using baited traps whilst fish are more often observed on baited cameras.

Current theories suggest that most amphipods form isolated populations cannot readily cross abyssal partitions.

With a few rare exceptions, this is also believed to be the case for dominant fish species sampled from depths below 6000 m. 

This would explain why some trenches are host to species endemic to that area and why we sometimes see populations evolve to become distinct species between neighbouring trenches, something which is more pronounced with increasing distance between trenches.

These studies highlight that, whilst non-subduction features are an overlooked minor fraction of the total hadal area, they are essential to our understanding of the ecological and evolutionary dynamics across the hadal zone.

Alan Jamieson,��marine biologist at Newcastle University and lead author of the study.

One thing our scientists know for certain is that their continued efforts to study seabed geomorphology is crucial to understanding the connectivity between hadal zones.

And we should continue to explore the mysteries of these hadal depths, because our relationship with even the deepest reaches of our maritime environment underpins life on earth.