Secrets of the seabed: The hidden landscapes beneath the waves

More than two thirds of the Earth’s surface lies beneath water, and it is down there that most of the planet’s landforms are found. Valleys, plains, ridges and canyons also exist in the ocean-often on a far larger, harsher and more unfamiliar scale than anything we see on land. Understanding the seabed is a direct route to understanding how our planet works, from climate and currents to fish stocks and natural resources.

Why the seabed is so crucial to our planet

Around 71% of the Earth is covered by water, and roughly 97% of that water sits in the oceans. In other words, the majority of the Earth’s topographic features are actually underwater rather than on land. Even so, scientists know the far side of the Moon better than many regions of the ocean floor.

The ocean’s deepest point drops further below sea level than Mount Everest rises into the sky.

It helps to picture this hidden world as anything but a uniform depth. The ocean floor forms a patchwork of shallow shelf seas, abrupt slopes, vast abyssal plains, hills, trenches and volcanic islands. Each of these shapes plays a distinct part in ecosystems, plate tectonics and the climate.

One reason so much remains unknown is simply access: the deeper you go, the darker, colder and higher-pressure the environment becomes. Modern seabed mapping relies on tools such as shipborne sonar, autonomous underwater vehicles and satellite measurements of sea-surface height, which can hint at major underwater structures. Even with these technologies, large areas are still mapped only in broad strokes.

Continental shelf (seabed): shallow seas with enormous importance

The gently sloping, near-shore zone that extends out beneath the sea is called the continental shelf. It is, in effect, the flooded edge of the continents. Water depth here typically stays below a few hundred metres, yet this zone still accounts for about 8% of the entire seabed.

How wide this fringe is varies dramatically:

• Off Siberia, the shelf stretches about 1,500 kilometres into the Arctic Ocean.
• Off large parts of Africa, it ends after only around 10 kilometres in the Atlantic.

From an ecological standpoint, the continental shelf is a hotspot for life. Suspended particles and nutrients arriving from land tend to accumulate here, and sunlight penetrates far enough to support photosynthesis. Estimates suggest the continental shelf is home to:

• around 90% of the world’s fish stocks

Because the continental shelf concentrates biodiversity as well as human activity (shipping, fishing and coastal development), it is also one of the most heavily pressured parts of the seabed. Measures such as marine protected areas, careful fisheries management and tighter controls on pollution and seabed disturbance can make a decisive difference-both for coastal ecosystems and for the communities that depend on them.