If you were to sail out into the open ocean and drop a glass marble over the side of the boat, for the first six or seven minutes it would fall through the uppermost layer of water, the part where the sun still shines.


Some call this the epipelagic or euphotic zone, or simply the sunlit zone. It’s the most familiar part of the oceans, where most of the known species live, and it’s where all the oceans’ photosynthesis takes place. The sun-catchers come in the form of large seaweeds as well as microscopic, single-celled creatures, collectively known as phytoplankton, which all suck in carbon dioxide and turn it into food for almost all the rest of ocean life.

As the marble drops, the sunlight fades until, at around 200 metres, there is just enough dim blue light to see by but not enough to power photosynthesis, and phytoplankton venture no deeper (at least not while they’re still alive).

Here the marble enters the deep. Below, horizontal zones are laid out one above the other, like layers of jelly poured into a tall sundae glass. The uppermost layer, reaching downward from 200 metres, is the twilight zone (also known as the mesopelagic).

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The falling marble takes almost a half hour to pass through this zone of indigo dusk, until, at 1,000 metres, the twilight zone gives way to the permanent dark of the midnight zone (or bathypelagic). At this depth, the temperature, which has been falling, begins to level off.

  • The Brilliant Abyss is one of our top science books for March 2021

So far, the marble has been passing through the thermocline, where seawater rapidly cools, from the sun-warmed surface into the oceans’ dark interior. Within the midnight zone, across most of the planet, water stays at a steady 4°C. It takes the marble another hour and a half to pass all the way through the midnight zone, at which point it reaches the next great portion of the deep, between around 4,000 and 6,000 metres down, the part of the deep sea now officially known as the abyss.

All through its journey towards the seafloor, the marble passes living creatures. Flashing lights glint in the glass sphere, not from sunlight but from the many light-making animals, the glowing worms and flashing lanternfish, which might wonder what kind of creature is blinking back at them.

The marble would collect a dusting of organic matter, and tiny shrimp might take a ride while scraping up those particles of food. In the open waters of the midnight zone, the marble could get knocked aside by the swish of a sperm whale’s tail as it chases after a squid. It might bounce down the steep rocky walls of a canyon or land in a soft abyssal plain, perhaps next to a herd of sea cucumbers that look like small, pale piglets with too many legs, some with a spiny, red crab riding on their backs because there’s nowhere else for it to hide.

The marble could land on the flank of a seamount and get lost in tangled forests of immobile animals that have been living there for centuries, or it could fall next to a scorching hot spring gushing from cracks at a mid-ocean ridge and land among clusters of giant clams and huge worms with scarlet feathers.

Or, if your aim was good, the marble could plunge down into a trench and reach the hadal zone, the oceans’ deepest layer. Even there, the glass sphere would pass living things, the blur of ghostly white fish. Then, eventually – six hours after you let it go at the surface – the marble would land at the very bottom, just over 11 kilometres down, where it might attract a hungry swarm of pale crustaceans, eager to have a go at eating it.

A total tally of the number of deep-sea species is, of course, a long way out of reach given the deep’s vast size, and systematic surveys have revealed glimpses of what is still to be found. In 1984, two American scientists, Fred Grassle and Nancy Maciolek, used a box corer, a tool like a giant cookie cutter, to extract chunks of mud from the deep seabed off the coasts of New Jersey and Delaware, between 1,500 and 2,500 metres down.

Carefully sifting through the mud and picking out every tiny living thing – every worm, crustacean, starfish, sea cucumber, clam and snail – they identified 798 species, over half of them new to science. Based on an average of three new species per 2.5 square kilometres of seabed, Grassle and Maciolek estimated that abyssal plains across the planet could be home to 30,000,000 species. The duo acknowledged that some regions of the deep may support a lower density of species, so they dialed down their estimate to a more cautious 10,000,000.

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More than 35 years after Grassle and Maciolek’s groundbreaking study, the task of knowing all that lives in the deep continues. In 2019, a team of 17 lead scientists published the results of a three-year survey of the Pacific in an area of deep sea bigger than the state of California, involving hundreds of hours of dive time using remote-operated submersibles.

In all, they photographed 347,000 animals, and only one in five of them were known species. Some were too small, or the pictures too blurry, to identify, but the majority were animals that nobody had ever seen before. The diversity of life is prolific in the deep, rivalling the shallow, familiar seas – and perhaps even life on land.


A central inventory of deep-sea life, the World Register of Deep-Sea Species, has been growing since 2012, a cataloguing job that is far from complete, as more species are constantly being added. By 2020, there were 26,363 listed species. All these organisms, and multitudes more besides, have evolved ways to survive and thrive in the extreme conditions of the deep, something that until relatively recently was thought to be impossible.

The Brilliant Abyss by Helen Scales is out now (£16.99, Bloomsbury Sigma).
The Brilliant Abyss by Helen Scales © Bloomsbury Sigma


Dr Helen Scales is a marine biologist, broadcaster and science writer. She is the author of Spirals in Time and The Brilliant Abyss.