Fanworms belong to a group of marine worms called polychaetes, relatives of the bristle worms that crawl through reefs and sediments. But unlike their wandering cousins, fanworms build protective tubes from mucus, sand, shell fragments, or calcium. Hidden inside these tubes, the worm’s soft body remains safely out of sight.

The seafloor is not a solid, impermeable barrier. Much of it is fractured and porous, allowing seawater to penetrate deep into the planet’s crust. Driven by heat from the interior, this water slowly circulates through networks of cracks and channels beneath the seabed. Some of these underground journeys may last thousands of years, and the water emerges altered by its passage, for it carries dissolved minerals and chemicals that influence the chemistry of the oceans above. Even below the floor of the sea, the ocean continues its hidden circulation.

Tiny, shrimp-like crustaceans, lanternfish, squid, gelatinous drifters, transparent hunters, a multitude of fish and many more. Together, they ascend towards the surface in immense numbers, coming with the protection of darkness to feed in the richer waters above. Then, before dawn, they sink once more into the dimness of the depths. This happens every night in oceans around the world, yet most people have never heard of it.

Beneath the calm surface of the ocean lies a world of immense geological energy—continents drifting apart, mountains rising, crust vanishing into the mantle. 

The ocean conceals a restless planet.

Earth’s broken shell

The outer surface of the Earth is not a single unbroken shell. It is divided into enormous pieces called tectonic plates. These plates drift atop softer, slowly flowing rock deep within the planet. Some move only a few centimetres each year, about as fast as fingernails grow.

Every day, divers capture images of fish living naturally on coral reefs, over sandy bottoms, in kelp forests, estuaries, caves, and the open sea. These photographs are far more than souvenirs or artistic images. Increasingly, they are valuable scientific records, documenting marine biodiversity at a time when aquatic ecosystems are changing rapidly.

Many bodies, one life

A siphonophore is composed of hundreds, sometimes thousands, of tiny individuals called zooids. Each zooid is genetically identical, yet specialised for a particular task. Some swim, some feed, some reproduce, and some have stinging cells that they use not only to kill prey, but also to defend the colony.

Though each zooid is a separate animal, none can survive independently. Over the abyss of time in which siphonophores evolved, they became so integrated that the colony functions as one organism.

This is methane hydrate, which consists of water molecules forming crystalline cages that trap methane gas inside. Vast deposits lie buried in sediments along continental margins and beneath Arctic seas, holding more carbon than all known fossil fuels combined.

And yet, despite their scale, they remain one of the least understood systems on Earth.

A world without sunlight

For much of human history, it was assumed that all life depended, directly or indirectly, on the sun. Plants absorbed sunlight through photosynthesis, herbivores ate the plants, and predators ate the herbivores. Each step in the food chain was based on solar energy.

But, in 1977, that notion was turned on its head.

Alive in ways we are only beginning to understand, it is pumping, filtering, sensing, repairing, coordinating. 

A sponge survives by moving water. Through thousands or even millions of tiny pores, it draws seawater into its body. Specialised cells beat microscopic hairs that create a continuous current, bringing oxygen and food particles inside. A sponge the size of a football can filter thousands of litres of seawater in a single day.

At the heart of this system lies one of the great mysteries of modern oceanography: the global “thermohaline circulation,” also known as the Atlantic Meridional Overturning Circulation, or AMOC. Scientists are still trying to understand just how stable it is.