Imagine a mountain range larger than the Alps, with peaks soaring thousands of meters high. Now, imagine this entire geological marvel hidden from sight, submerged in the crushing pressure and perpetual darkness of the deep ocean. These are not features of a science fiction novel, but very real—and vital—parts of our planet’s geography. Welcome to the world of seamounts.

What is a Seamount? The Geography of the Deep

In the simplest terms, a seamount is an underwater mountain. The formal geographic definition classifies a seamount as a distinct elevation rising at least 1,000 meters (3,281 feet) from the surrounding seafloor without ever breaching the water’s surface. If it breaks the surface, it becomes an island. If it once broke the surface and has since been eroded to have a flat top, it’s called a guyot.

The vast majority of seamounts are volcanic in origin, born from eruptions on the ocean floor. They form in several ways:

• Hotspots: Just like the volcanoes that formed the Hawaiian Islands, a stationary plume of magma in the Earth’s mantle can punch through a moving tectonic plate, creating a chain of volcanoes. As the plate moves, older volcanoes are carried away from the hotspot and sink beneath the waves, becoming seamounts. The Hawaiian-Emperor seamount chain in the Pacific is a classic example of this process.
• Mid-Ocean Ridges: Many seamounts form near the boundaries of tectonic plates, where seafloor spreading occurs.
• Subduction Zones: They can also form where one tectonic plate is forced beneath another.

While we have mapped the surface of Mars in high resolution, it’s estimated that less than 20% of our own planet’s seafloor has been mapped to a similar detail. Scientists estimate there are over 100,000 seamounts taller than 1,000 meters across the globe, forming the most common and extensive habitat on Earth. They are silent, hidden worlds that profoundly shape the ocean around them.

Islands of Life in an Ocean Desert

The deep, open ocean is often compared to a desert—vast, sparsely populated, and with limited food. Seamounts, however, are the complete opposite. They are vibrant oases, teeming with life and considered hotspots of marine biodiversity. This incredible productivity is due to a phenomenon known as the “seamount effect.”

As deep, slow-moving ocean currents encounter the massive obstacle of a seamount, they are forced upwards. This process, called upwelling, carries nutrient-rich water from the abyssal depths towards the shallower, sunlit (or at least less dark) waters. These nutrients, such as nitrates and phosphates, fuel the growth of phytoplankton, which form the base of the marine food web.

But the benefits don’t stop there. The hard, rocky slopes of a seamount provide a crucial foundation on an otherwise soft, muddy seafloor. This solid ground is the perfect real estate for sessile, or attached, filter-feeding organisms. Ancient, slow-growing deep-sea corals and giant glass sponges can gain a foothold, forming intricate, three-dimensional forests and gardens. Some of these coral colonies can be thousands of years old, creating habitats of staggering complexity.

This complex structure provides food and shelter for a huge diversity of other animals. Crabs, lobsters, starfish, sea anemones, and countless species of fish thrive in these coral gardens. Because of their isolation, many seamounts also have a high degree of endemism, meaning they host species found nowhere else on Earth. The Davidson Seamount, a well-studied volcanic mountain off the coast of California, is a designated “Seamount Marine Conservation Area” precisely because it is an “oasis in the deep”, home to vast coral forests and rare sponge fields.

How Seamounts Shape the Seas

The influence of seamounts extends far beyond their own slopes. Their sheer physical presence alters the fundamental patterns of ocean circulation. By blocking and steering currents, these underwater mountains create complex systems of eddies, vortices, and internal waves that can persist for hundreds of kilometers.

These eddies act like giant, slow-spinning whirlpools that trap the nutrient-rich water and plankton brought to the surface by upwelling. This creates reliable, concentrated feeding stations in the open ocean. Large, migratory predators—including tuna, billfish, marine mammals like whales, and sharks—have learned to navigate by these undersea landmarks, using them as crucial rest-stops and cafeterias on their vast oceanic journeys.

In this way, the physical geography of a single hidden mountain can influence the biological and human geography of an entire ocean basin, determining where fish populations aggregate and, consequently, where fishing fleets travel.

A Fragile World Under Threat

Despite their remoteness, seamount ecosystems are exceptionally vulnerable to human activity, primarily from a destructive fishing practice called deep-sea bottom trawling.

Because seamounts attract and concentrate commercially valuable fish species like the orange roughy and Patagonian toothfish (often sold as Chilean sea bass), they have become prime targets for industrial fishing fleets. Bottom trawling involves dragging enormous, weighted nets across the seafloor. These nets, often flanked by heavy steel plates or massive rubber wheels, are indiscriminate. They pulverize everything in their path.

The effect on a seamount is catastrophic. It is the marine equivalent of clear-cutting an old-growth redwood forest to catch a few squirrels. The ancient, fragile coral gardens and sponge beds—which took millennia to grow—can be obliterated in a single pass of a net. The complex habitat is reduced to rubble, and with it, the entire ecosystem it supports collapses.

Compounding the problem is the biology of the fish being targeted. Species like the orange roughy are extremely long-lived (up to 150 years) and slow to reproduce. They gather in large numbers around seamounts to spawn, making them easy to catch in massive quantities. This combination of habitat destruction and targeting slow-growing species has led to the rapid collapse of many seamount fisheries and the potential extinction of unique species before they are even discovered.

Emerging threats, such as deep-sea mining for the cobalt-rich mineral crusts that form on seamount slopes, pose a further risk to these irreplaceable habitats.

Protecting the Ocean’s Hidden Worlds

The good news is that awareness is growing. Scientists and conservationists are working to identify and study these critical ecosystems. The establishment of large-scale Marine Protected Areas (MPAs) is one of the most effective tools for their preservation. For example, the Papahānaumokuākea Marine National Monument in the Northwestern Hawaiian Islands protects a vast area of the Pacific, including a long chain of seamounts.

These hidden worlds are not just geological curiosities; they are engines of ocean life, drivers of currents, and archives of our planet’s history. They remind us that the ocean’s health depends as much on the unseen as it does on the familiar. Protecting them is not just about saving deep-sea coral—it’s about preserving the integrity of the entire global ocean system.