How undersea mountains shape life in deep

The vast, rolling seafloor of Earth is dotted with more than 100,000 volcanic seamounts, towering undersea mountains that act as biodiversity hotspots and magnets for sharks. These long-extinct volcanoes rise at least 1,000 metres from the seabed, sometimes forming ridges, plateaus, or even islands when their peaks reach the surface.

As the BBC describes, these undersea mountains form “borderlands” where deep-sea species mingle with open-ocean life, creating ecosystems far richer than the surrounding flat seafloor.

Although fewer than 0.1% of seamounts have been explored, scientists are uncovering their ecological importance at an accelerating pace.

“An unprecedentedly large number of volcanic seamounts have been found on the seafloor over the past couple of decades,” says Ali Mashayek of the University of Cambridge, speaking to the BBC. One such region lies around Ascension Island in the mid-Atlantic, the exposed peak of a vast volcanic chain stretching toward Africa.

Marine ecologist Sam Weber, cited by the BBC, spent years studying three nearby seamounts: the deep Harris Stewart seamount and two shallower “Southern Seamounts,” Grattan and Young. Working from a longline vessel alongside commercial fishers, Weber and colleagues tagged sharks in an unexpectedly predator-rich environment.

On the Southern Seamounts, the team found 41 times more shark biomass than in surrounding waters, along with five times the diversity and thirty times the overall biomass of sharks and large predatory fish. Threatened species, including silky sharks, were particularly abundant.

“Sharks, including deep-sea and migratory sharks, love to be around seamounts – across all the global oceans,” notes ocean governance expert Lydia Koehler in the BBC piece.

Yet the reasons remain uncertain. Some hypotheses suggest seamounts provide shelter or food, or act as geomagnetic waypoints. Seamounts possess unique magnetic signatures because their iron-rich volcanic rock “freezes” the Earth’s magnetic field in place as it cools.

Weber tells the BBC, “Seamounts have a really strong magnetic signature that a lot of these species are attuned to.”

Two leading explanations—the “oasis” and “hub” hypotheses—describe how seamounts draw predators. As Walter Munk famously described them, they are “the stirring rods of the ocean,” generating turbulence that drives deep nutrient-rich water upward, fuelling life. Yet Weber found limited evidence of increased phytoplankton around Ascension.

Instead, seamounts appeared to concentrate vertically migrating mid-water species, creating focal points of food through “trophic focussing.” Sharks and tuna may then visit the mountains as gathering places or energy-saving refuges. Weber’s favourite interpretation, the “service station hypothesis,” proposes that upwelling waters allow sharks to rest while still swimming.

“The fact that they can’t stop swimming incurs an energetic cost,” he explains.

The impacts of seamounts extend outward in a “halo,” sometimes as far as 40 kilometres, where enhanced predator abundance persists. Sharks tagged around Ascension often left to forage hundreds of kilometres away but returned repeatedly, suggesting the seamounts serve as home bases.

However, the same features that attract sharks make seamounts vulnerable to intense fishing pressure. Bottom trawling and longline fisheries can devastate these slow-growing ecosystems.

“Bottom trawling demolishes the benthic ecosystem,” warns Weber.

Conservation momentum is growing: Ascension’s Exclusive Economic Zone was closed to commercial fishing in 2019, and in 2025 the IUCN adopted a motion urging a global phase-out of bottom trawling on seamounts.

By Sabina Mammadli