Unveiling the Ocean's Hidden Giants: The Undersea Mountains Where Sharks Rule

Exploring Earth's Hidden Underwater Peaks

Beneath the vast, undulating expanse of the world's oceans lies a breathtaking secret: colossal underwater mountains. These "seamounts," like submerged constellations, dramatically punctuate the seafloor, providing a bustling sanctuary for an astonishing array of marine life, including a surprisingly high concentration of sharks.

Imagine ocean currents roaring across the summits of long-extinct volcanoes, far below the surface. These majestic formations rise steeply, often soaring over 1,000 meters (3,300 feet) from the abyssal plains. Some are pockmarked with craters or ridged with ancient lava flows, while others boast expansive, flat plateaus. Occasionally, their lofty peaks even break the surface, forming isolated islands that hint at the grandeur hidden beneath.

Seamounts represent critical borderlands where the deep ocean meets shallower waters. Here, a unique convergence of reef-dwelling and open-ocean species occurs across every tier of the food chain, from microscopic plankton feeders to formidable apex predators. Each seamount is a distinctive ecosystem, brimming with corals, crustaceans, sponges, sea stars, fish, octopus, turtles, whales, dolphins, and, notably, sharks. Compared to the barren flat seafloor, these underwater peaks harbor a significantly greater abundance and diversity of creatures, many of which are found nowhere else on Earth. And, remarkably, countless more remain undiscovered.

Globally, an estimated 100,000 such undersea mountains are scattered from the icy North Atlantic to the tropical Pacific. Yet, astonishingly,fewer than 0.1% have ever been explored. "An unprecedentedly large number of volcanic seamounts have been found on the seafloor over the past couple of decades," notes Ali Mashayek, an associate professor of climate dynamics at the University of Cambridge.

The Astonishing Discovery: Sharks Reign on Undersea Mountains

As ocean exploration accelerates, new seamounts are continuously coming to light. A recent expedition focused on Atlantic Ocean seamounts yielded a particularly intriguing discovery: an extraordinary concentration of the ocean's top predators – sharks.

Ascension Island: A Window into Shark Sanctuaries

Sam Weber, a lecturer in marine vertebrate ecology and conservation at the University of Exeter, spent seven years as principal conservation scientist on Ascension Island, describing it as "a tiny dot of green" halfway between Africa and Brazil. This remote speck of land is, in fact, the visible tip of an underwater volcano, part of a vast seamount chain stretching for hundreds of miles across the Atlantic.

Weber's team investigated three seamounts approximately 300km (186 miles) off Ascension's coast. The Harris Stewart seamount, for instance, reaches only the "twilight zone" (200-1,000m deep), bathed in perpetual dusky light. In stark contrast, the sister "Southern Seamounts" of Grattan and Young are shallower, their peaks rising to within 100m (330ft) of the surface, just 80km (49.7 miles) apart. These distinct environments offered unique insights.

Unprecedented Shark Numbers

Aboard a longline fishing boat, collaborating with commercial fishermen, Weber and his colleagues caught and tagged numerous sharks. It was on these Southern Seamounts that they made a truly remarkable finding:41 times more sharks, in terms of biomass, compared to the surrounding open ocean. Beyond sheer numbers, these sites supported five times higher diversity and 30 times higher biomass of sharks and large predatory fishes combined. This included a significant presence of threatened or near-threatened species like silky sharks, alongside commercially valuable species such as yellowfin and bigeye tuna.

"Sharks, including deep-sea and migratory sharks, love to be around seamounts – across all the global oceans," affirms Lydia Koehler, an expert in ocean governance and associate lecturer at Plymouth University School of Biological and Marine Sciences.

Decoding the Mystery: Why Sharks Gravitate Towards Seamounts

The question of why these specific undersea mountains where sharks rule remains somewhat enigmatic. Scientists hypothesize that animals might visit seamounts for shelter, cleaning stations, abundant food, or even as crucial navigational landmarks.

Magnetic Signatures and Migratory Highways

Seamounts are formed from volcanic rock containing tiny iron atoms. As this rock cools, these iron atoms align with Earth's magnetic field, locking in a unique geomagnetic signature. Weber suggests that both whales and sharks might use these distinct magnetic fields as "waymarkers." "Seamounts have a really strong magnetic signature that a lot of these species are attuned to," he explains, pondering if these formations act as "stepping stones along their migratory corridor."

The Oasis and Hub Hypotheses

Two primary theories attempt to explain this predator-aggregating effect: the oasis hypothesis and the hub hypothesis.

"The Stirring Rods of the Ocean"

In 1998, pioneering oceanographer Walter Munk famously dubbed seamounts "the stirring rods of the ocean." This vivid description helps explain how a seamount can transform into a nourishing oasis. When powerful ocean currents collide with the steep slopes of an undersea mountain, water is forced upwards and over the summit. This process, known as upwelling, thrusts cold, nutrient-rich deep water towards the sunlit surface.

"We have realized that the turbulence generated by the interaction of oceanic deep flows with these seamounts plays a significant role in driving deep ocean circulation," states Mashayek. In fact, his team's research indicates that this seamount-induced stirring contributes to approximately one-third of all global ocean mixing. Laura Cimoli, a physical oceanographer at the University of Cambridge, adds that this mixing "exerts control on the deep ocean storage of natural and anthropogenic properties – such as nutrients, oxygen, heat and carbon."

The sudden influx of nutrients near the surface triggers blooms of phytoplankton, the foundation of ocean food chains. This process transforms the seamount into a fertile, food-rich oasis, which in turn attracts larger predators like sharks.

Beyond Upwelling: Trophic Focussing and Service Stations

While upwelling clearly plays a role, Weber's study at the Ascension seamounts found little evidence of increased primary productivity from phytoplankton. Instead, the higher biomass of animals in the lower and mid-levels of the food chain was primarily driven by vertically migrating mesopelagic species – creatures that ascend from the twilight zone.

Weber explains that filter feeders and other marine life benefit from prey being concentrated at the peak, a phenomenon called "trophic focussing." "Seamounts can act like giant traps for planktonic organisms that are blown across it constantly," he says. As plankton moves up and down, the seamount's summit can trap it, allowing this energy to "cascade up through the food web" to all creatures above.

While some animals are "resident," calling a particular seamount home, Weber's findings suggest that species like Galapagos and silky sharks, along with yellowfin and bigeye tuna, often gather at seamounts after foraging in the open ocean. "The energy is generated off the seamount and the foragers – the mobile predators – go and collect it and bring it back." This aligns with the "hub theory," where seamounts serve as vital places to gather, socialize, mate, rest, or as a home base after extensive hunting.

Weber's favorite explanation, which he calls the "service station hypothesis," posits that specific spots on seamounts, perhaps thermal updraft areas, allow sharks to "swirl and save energy." Many shark species must swim continuously to pump water over their gills, incurring an energetic cost. Similar to an eagle soaring on thermals, upwelling currents around seamounts could enable sharks to float and conserve energy when not actively feeding, before heading "back off and feed in the blue again."

One tagged silky shark, for instance, left the seamount nightly to forage up to 100km (62 miles) away, yet consistently returned. "They can obviously navigate to these seamounts very effectively," Weber notes, suggesting these sites are crucial checkpoints on longer journeys. The mysterious figure-eight pattern of its foraging trips, while unexplained, further supports the hub theory. The temporary disappearance of some tagged silky sharks for about a year reinforces the idea that seamounts act as a "service station" where they "check in, maybe feed, then carry on their migration."

It appears that both the oasis and hub theories likely apply, with seamounts supporting a complex mix of resident populations, regular visitors, and transient species.

The "Halo Effect": Seamounts' Far-Reaching Influence

The influence of these remarkable habitats extends far beyond their summits. A "halo" of increased marine life has been observed spreading up to 6km (3.7 miles) into the open ocean from the peaks, with other studies suggesting this effect can reachas far as 40km (25 miles). "You can go kilometres away [from the summit] and still find higher predator abundance," Weber explains, even in areas thousands of meters deep.

Silky sharks, yellowfin, and bigeye tuna travel hundreds of kilometers from seamount summits but seem to remain "site-attached." Acoustically tagged Galapagos and silky sharks even journeyed the 80km (50 miles) between the two Southern Seamounts, treating both as home. This "halo" of enhanced predator abundance expands and contracts with night and day, as silky sharks move off the summits at night only to return at dawn. "So, it's a zone of enrichment, and they connect to each other. When you're thinking about protected area design, you need to be thinking bigger than just the summit itself," emphasizes Weber.

Protecting These Vital Underwater Sanctuaries

Evidence is rapidly accumulating, underscoring seamounts as irreplaceable oases of biodiversity in the deep ocean, critically important for elasmobranchs – a group encompassing sharks, rays, and skates – many of which are among the planet's most endangered animals. "Sharks are a crucial part of the entire marine ecosystem, from shallow to deep, from coastal to offshore," states Koehler, highlighting the unique role each of the thousand-plus species plays.

The Dire Threat of Overfishing

However, the very aggregation of life that makes seamounts so vital also makes them vulnerable. For decades, these underwater mountains – where commercially desirable fish like bigeye tuna are readily found – have been targets for industrial fisheries, especially through bottom trawling.

"The biggest threat to sharks is overfishing," warns Koehler. This includes both targeted fishing andbycatch, where sharks are unintentionally caught but still landed and sold. Bycatch is one of the five greatest threats to ocean life, with sharks among the most susceptible. The grim reality is that hundreds of millions of sharks are estimated to be killed by commercial fisheries annually.

Bottom trawling, a particularly destructive fishing method, involves dragging a heavy, weighted net across the seafloor. "You can imagine how much damage that does to the fine sponges, the corals, all those really fragile ecosystems. You basically remove them. And what you've got left is just a bare field of nothing," Koehler explains. Everything in the net's path is scraped away, annihilating the entire benthic ecosystem and food web. Compounding the issue, many seamount species are slow-growing with long life histories, making them incredibly vulnerable to rapid, "boom-bust" fisheries that deplete populations in short order.

The impact of bottom trawling on seamounts can last for decades, even centuries. "It will take more than a human's lifetime – more than multiple lifetimes – for these ecosystems to regenerate and to recover," Koehler stresses. "And in the meantime, you remove these really important services: food sources, nursery grounds. They are not functional anymore for those species that really depend on them."

A Path Forward: Conservation in Action

Despite the threats, knowing precisely where sharks gather provides vital knowledge for advancing conservation. In 2024, Emanuel Gonçalves, chief scientist at the Oceano Azul Foundation, led an expedition to the Gorringe Seamount off Portugal. Following this, the Portuguese government declared a new protected marine area. As Gonçalves aptly puts it, "Seamounts are unique and need to be protected."

Similarly, the entire Exclusive Economic Zone (EEZ) around Ascension Island was closed to commercial fishing in 2019. The United Nation's Second World Ocean Assessment in 2021 explicitly stated, "Fishing, especially bottom trawling, constitutes the greatest current threat to seamount ecosystems." Further cementing this, the International Union for Conservation of Nature (IUCN) adopted "Motion 032" in October 2025, calling for a global transition away from bottom trawling on seamounts by the end of 2026.

However, Weber cautions that preventing bottom trawling alone is insufficient, as top predators are also frequently caught by pelagic longlines. "Predators are the species that have been hammered most by commercial fishing," he asserts. "I would definitely be supportive of having at least some seamounts shut down altogether, like we've done in Ascension Island. Get rid of trawling, get rid of longline fishing. Protect the whole ecosystem."

These majestic undersea mountains are not merely geological formations; they are dynamic, life-sustaining pillars in the deep ocean, and critical habitats for the majestic predators that roam our seas. Protecting these vital underwater sanctuaries is paramount for the health of our global marine ecosystems.