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Orange-lined triggerfish (Balistapus undulatus), Red Sea, Egypt. This demersal triggerfish has a maximum body size of about 30 centimetres (12 in) and is found across the Indo-western Pacific East Africa, Red Sea and Japan, typically in coral reefs ecosystems, coral lagoons, and external reef slopes within this area. Within coral reefs, the orange-lined triggerfish are more versatile than other triggerfish species and can be found at depths up to 50 metres (160 ft), although studies have found that they prefer depths of 2 to 8 metres (6 ft 7 in to 26 ft 3 in). It's an omnivorous feeder that can, because of its strong and heavy teeth, tackle a variety of benthic prey like cnidarians, molluscs, ctenophores, crustaceans, other fish, algae, and echinoderms. This species in particular, given its broad diet and distribution, is a crucial component in coral reef ecosystems through top-down control and especially through consumption of sea-urchin.

Diego Delso, CC BY-SA 4.0, via Wikimedia Commons. View source.

NOAA on Unsplash

NASA Astronomy Picture of the Day:

What’s causing this unusually long cloud on Mars? Known as the Arsia Mons Elongated Cloud (AMEC) and first recorded by NASA’s Viking Orbiter 2 in 1977, the strange cloud has been imaged in greater detail more recently by ESA’s robotic Mars Express spacecraft. The featured image is from 2018. The cloud forms only during spring and summer seasons and then only during morning hours. Made of water-ice, the most familiar process that causes cloud crystals to form around bits of dust would fail due to the thinness and lack of dust high in the Martian atmosphere. Leading speculation holds that dust is not involved. After the cloud forms near the volcano Arsia Mons, wind blows the cloud downstream — creating a tail that can extend over 1500 kilometers. Efforts to better understand AMECs will continue, an understanding that may bolster a more complete view of cloud formation on Earth.

Photo by ESA, DLR, FU Berlin; Processing & License: J. Cowart

The Ocean’s Hidden Landscape || Kudos365 - Exploring Our World Series.

Overview

Beneath the water lies a changing world of mountains, canyons, plains, and trenches that helps explain how Earth works. Stand on a beach and the ocean appears to begin where the land ends. The surface stretches toward the horizon, concealing almost everything beneath it. Yet the shoreline is only where the ground disappears from view. Continue outward underwater and you may cross a submerged continental platform, descend a steep slope, pass through a canyon, and reach a plain several miles below the waves.

Farther away, mountains rise from the darkness. Some form long volcanic ridges; others stand alone. In certain regions, the seabed bends into trenches deeper than Mount Everest is high. These features are connected by processes that build rock, move it across the planet, break it apart, and bury it. The useful question is therefore more than what the ocean floor looks like. It is why that landscape has its particular shapes—and how we can know when so much of it remains hidden beneath deep water.

Learning to see beneath the surface

Early depth measurements used weighted lines lowered from ships. Each sounding supplied a depth at one location, leaving the ground between measurements largely uncertain. Sound-based instruments eventually allowed ships to measure depths more efficiently, revealing underwater relief on a much larger scale.

Those discoveries helped change geology. During the twentieth century, evidence from submarine ridges, earthquakes, magnetic patterns in seabed rocks, and the ages of those rocks contributed to the development of plate tectonics. The ocean floor became evidence that Earth’s outer shell moves. What had been difficult terrain to measure also became a record of how the planet continually reshapes itself.

Key Terms at a Glance

Continental shelf: The continent’s shallow underwater edge.

Submarine canyon: A valley carrying material toward deeper water.

Abyssal plain: A broad, relatively flat stretch of deep seabed.

Mid-ocean ridge: A mountain system where new oceanic crust forms.

Seamount: A mountain rising from the seabed.

Ocean trench: A deep depression where a plate bends downward.

Hydrothermal vent: An opening where heated water returns to the ocean.

Bathymetry: Measuring and mapping underwater depths.

The continent continues underwater

Our imagined journey begins on the continental shelf, the relatively shallow, gently sloping submerged edge of a continent. Its width varies enormously. In some places, deep water lies close to shore; elsewhere, the shelf extends far out to sea.
This explains why distance from a beach is a poor way to judge depth. A vessel can travel a considerable distance over a broad shelf while remaining above comparatively shallow ground. Along a narrow margin, it may encounter deep water much sooner.

Beyond the shelf, the continental slope descends toward the deep ocean. Where sediment accumulates at its base, it can build a gentler region called the continental rise. This arrangement is common along some margins, but it is not a universal staircase. At active plate boundaries, trenches and other structures can replace that simple sequence. Coastlines mark the water’s edge; they do not consistently mark the edge of a continent’s underlying crust.

Canyons carry material into deep water

A canyon can interrupt the descent. Submarine canyons cut into continental margins, sometimes beginning near shore and sometimes far offshore. Their branching channels can resemble valleys on land, but resemblance alone does not establish how they formed.
Underwater landslides and sediment-carrying flows help excavate and enlarge them. When enough sediment becomes mixed with water, the mixture can become denser than surrounding seawater and move downslope under gravity. Such movements, called turbidity currents, can transport sand and mud into deep water. Some also mobilize substantial material from the seabed itself.

The comparison with a river is useful only up to a point. A submarine canyon does not require an ordinary river flowing through air, and many were not simply dry valleys drowned by rising seas. Their histories can combine erosion, slope failure, changing sea levels, and connections to rivers. They are routes through which coastal material reaches the deep ocean.

Sediment smooths parts of the deep seabed

At the bottom of the slope, broad, relatively level stretches may extend across the deep basin. These are abyssal plains, commonly found several kilometers below the surface. Their smooth appearance can conceal a much rougher rocky foundation.
Imagine filling the low places in a crumpled surface with fine powder. The top becomes smoother even though the original relief remains underneath. Sediment can have a similar leveling effect on seabed rock, although its transport and accumulation involve several processes rather than one even sprinkling. Material arrives through sediment flows and by particles settling through the water.

Not every deep region is flat or thickly covered. Hills, exposed rock, and other relief interrupt the seabed. Even the proportion classified as abyssal plain depends on how strictly researchers define flatness. The important idea is that the surface we map can reflect both the rock originally formed there and the material deposited over it.

Underwater ridges mark where new crust forms

Eventually our journey reaches rising terrain. The connected mid-ocean ridge system runs roughly 65,000 kilometers, or 40,000 miles, through the ocean basins. Much of it is submerged, making Earth’s longest mountain system largely invisible from the surface.
Along these spreading centers, tectonic plates move apart and molten rock rises, cools, and adds new oceanic crust. A plate includes the crust and a rigid portion of the upper mantle beneath it. The moving pieces are therefore thicker than the thin surface layer we might imagine from a cracked eggshell.

The ridge is elevated partly because the underlying material is hot. As oceanic lithosphere moves away, it cools, becomes denser, and generally subsides. This helps explain why broad ocean basins tend to deepen away from spreading ridges. The mantle beneath the plates is mostly solid rock capable of slow deformation; the plates do not sail across a worldwide ocean of liquid magma.

Volcanoes build mountains that may never become islands

Away from the main ridges, individual mountains rise above the surrounding seabed. Many of these seamounts are volcanoes built by repeated eruptions. A mountain can be substantial while its summit remains far below the ocean surface.
An island is therefore an incomplete view of a much larger structure. Where volcanic construction reaches above sea level, the exposed land represents only the upper part. Elsewhere, the entire mountain remains submerged.

Some underwater mountains have broad, flattened tops and are called guyots. Many preserve a history in which an island’s summit was worn down near sea level before subsidence carried it into deeper water. Their shape records a sequence of construction, erosion, and sinking. A flat top is evidence to investigate, rather than proof that every such feature followed an identical history. Understanding a seamount requires measurements of its form together with geological evidence about its age and composition.

Trenches mark places where plates descend

The deepest seabed occurs in long, narrow trenches associated with subduction, where one tectonic plate bends beneath another and descends into the mantle. Challenger Deep in the Mariana Trench lies about 11 kilometers, or nearly seven miles, below sea level. Exact depth estimates vary with measurement and analysis. A trench is not a bottomless crack opening through Earth. It is a depression in the surface associated with a plate bending downward. The descending plate remains solid to considerable depths.

At some subduction boundaries, plates can lock together while their continued motion builds strain. Sudden slipping releases earthquakes, including the largest kinds known. If an event displaces a large volume of seawater, it can generate a tsunami. Underwater landslides can also produce tsunamis. The danger depends on the movement and displacement involved; a deep trench by itself does not continuously produce destructive waves.

Hot water changes rock beneath the seabed

The landscape also contains places where water circulates through fractured rock, heats up, reacts chemically, and returns to the ocean. These hydrothermal systems connect seawater with heat inside Earth, often in volcanically active regions. Some vents build mineral chimneys. Hot fluid carrying dissolved material meets cold seawater, and minerals precipitate, producing deposits around the opening. The dark plume of a black smoker contains tiny mineral particles; its smoke-like appearance does not mean that an underwater fire is burning.

A map can reveal a volcanic ridge without showing whether a particular fracture is venting today. Researchers need other evidence, including water measurements, temperature readings, samples, and observations near the bottom. This is a useful distinction throughout ocean exploration: the shape of the ground tells part of the story, while its chemistry and present activity require additional investigation.

Sound reveals the ground that light cannot

How can researchers construct this landscape without draining the sea? A sonar instrument sends sound into the water and records returning echoes. With knowledge of sound speed, the travel time helps determine distance. Modern multibeam systems send beams across a swath beneath a vessel, collecting many depth measurements as it moves.
Sound speed varies with conditions, so accurate surveys require corrections rather than a single universal conversion. Ship position and motion also matter. The resulting study of underwater depths and relief is called bathymetry.

Satellites provide a different kind of evidence. Radar measurements of sea-surface height help reveal subtle gravitational effects associated with large underwater features. A seamount’s mass can slightly alter the sea surface above it. Researchers use these patterns to infer broad seabed structure; the radar does not pass through kilometers of water to photograph the bottom. Satellite estimates and direct soundings complement each other, but they provide different levels of detail.

A complete-looking map can still contain gaps

In its 2026 global grid release, GEBCO reported that 28.7 percent of the ocean floor had been mapped to modern standards. That figure does not mean the rest is an entirely blank space. Broad estimates already show much of the planet’s major underwater relief, while detailed direct measurements remain incomplete.
Think of knowing that a mountain range occupies a region without having a reliable map of its smaller valleys and slopes. That comparison captures the difference in detail, although marine mapping also involves distinct instruments and depth-dependent standards.

Nor does mapped mean photographed, sampled, or fully understood. A good depth survey can identify a feature worth investigating without establishing its composition or history. Robots and other instruments then provide closer observations. The scale of the ocean, survey time, operating costs, and difficult conditions all make closing the gaps a substantial undertaking.

How the landscape connects to our lives

Together, these features describe a working system. Ridges add crust, plate movement carries it away, and subduction returns oceanic material to Earth’s interior. Meanwhile, erosion and sediment transport reshape the surface, while volcanic activity and circulating hot water alter rock. The terrain records overlapping histories rather than a single construction event. Its shape matters to people far from research ships. Seabed surveys help engineers plan routes for communication cables and assess where steep slopes or powerful sediment movements could threaten them. Depth information also supports navigation and offshore construction.

Seafloor relief influences the movement of water above it. Ridges and passages can steer deep flows, while rough terrain contributes to mixing. Since ocean circulation transports heat, the underwater landscape is one part of the physical system relevant to climate. Understanding it requires combining maps with measurements of moving water, rather than assuming that terrain alone determines the currents.

What remains to be discovered

The broad explanation of spreading ridges, moving plates, and subduction is well established. Many local histories remain uncertain. A newly mapped mountain may require rock samples to determine when it formed. A canyon’s shape may leave open which episodes of erosion or collapse mattered most. An apparently quiet volcanic area may need repeated observations to reveal change.

Researchers also face a practical problem: a map is a record made at a particular time. Eruptions, earthquakes, landslides, and mobile sediment can alter the seabed. Improving coverage and detecting change are related but different tasks. Better instruments and autonomous vehicles extend what researchers can measure, while interpretation still depends on combining several kinds of evidence.

The Bigger Picture

The ocean’s smooth surface hides a landscape as varied as the land we recognize—and one that helped reveal why continents move. Its mountains, plains, and trenches are consequences of heat, moving rock, gravity, water, and time.

The next time the sea seems to stretch into an undifferentiated blue distance, imagine the ground continuing beneath it: first the submerged continental margin, then the deep basin and the structures beyond. We already understand the main processes that shape this world. Mapping and exploring it in detail is still unfinished work.

This article is for general educational purposes. Scientific understanding may change as new research becomes available.

Exploring Our World | Kudos365 © 2026. All Rights Reserved. | M1.0.4 | HSR2.4.

Saint-Charles Cemetery is the largest cemetery in Quebec City. Today is All Souls' Day.

Wilfredor, CC0, via Wikimedia Commons. View source.

Will Durant explains,  "The essential causes of Rome's decline lay in her people, her morals, her class struggle, her failing trade, her bureaucratic despotism, her stifling taxes, her consuming wars"

(Source: Will Durant, Caesar and Christ, p. 665).

Abraham Lincoln (February 12, 1809 - April 15, 1865)

 Zoltan Tasi on Unsplash

NASA Astronomy Picture of the Day:

What color is the Moon? It depends on the night -- and the time. Outside of the Earth's atmosphere, the dark Moon, which shines by reflected sunlight, appears a magnificently brown-tinged gray. The featured image highlights a collection of apparent colors of the full moon on Earth, documented by one astrophotographer over 10 years from different locations across Italy. A red- or yellow-colored moon usually indicates a moon seen near the horizon. There, some of the blue light has been scattered away by a long path through the Earth's atmosphere, sometimes laden with fine dust. A blue-colored moon is more rare and can indicate an atmosphere carrying larger dust particles. What created the purple moon is unclear -- it may be a combination of several effects. The last image captures the total lunar eclipse of 2018 July -- where the moon, in Earth's shadow, appeared a faint red -- due to light refracted through air around the Earth. Have you ever seen an unusually colorful Moon?

Photo by Marcella Giulia Pace