14 things you just have to know about the abyss

The abyss is synonymous with mystery, depth, darkness, strange animal species and ocean trenches 11,000 metres below the surface of the ocean.

• Abyssus abyssum invocat!*

The word "abyss" comes from the Greek word abyssos, meaning "bottomless". This is because for a long time, humans regarded the ocean as an unfathomable void.

*Do you remember this maxim uttered by the pirate in Asterix and Cleopatra, who was poised to sink once again?

• Where does the abyss begin?

The abyss is the deepest part of the ocean where no light ever penetrates (at depths of 2,000 m or more). The abyssal plains that make up most of the ocean floor lie between 3,000 and 6,000 metres below the surface.

• 10,912 m: the deepest point in the ocean

However, on six occasions, humans have managed to descend to the depths of the ocean, in the Mariana Trench to the east of the Philippines. They dived to a depth of almost 11,000 metres.

Who are these heroes who delved into the depths of the sea? In 1960, the Swiss Jacques Piccard and the American Don Walsh descended in the bathyscaphe Le Trieste. In 2012, James Cameron, director of Titanic and Abyss, also explored the depths of the ocean aboard the Deepsea Challenger. The American explorer Victor Vescovo broke the record by descending to 10,928m in 2019. In 2020, Chinese scientists took their turn diving into the depths of the ocean.

• Plastic is everywhere, even down at 11,000 m

Even the depths are not spared from pollution. A plastic bag was found in the Mariana Trench. 89% of the objects found are made of plastic, often disposable items such as cutlery.

• Coral in the abyss!

It's astonishing! Corals can be found in the deep sea. These corals, which are known as ingenious species, create a new living environment through their activity and serve as a habitat for many other species such as young fish.

• Is anyone down there?

Living conditions in the depths of the ocean are extreme: no light, very little food, freezing temperatures and overwhelming pressure! However, they do not prevent life from developing there: fish, bacteria, jellyfish, invertebrates, sharks, etc. can all be found there.

• And one, and two.... and 10 million!

Ten million - this is the number of species, from bacteria to the largest marine animals, that could exist in the abyss. Biological diversity with fascinating potential that remains to be discovered. Barely 2% of the ocean floor has been explored by submarines.

• Life in the abyss

To live in such extreme conditions, adaptation is essential. With protruding eyes to capture the little light available, bioluminescence to communicate or hunt, or even to camouflage themselves, and enormous jaws with sharp teeth, the animals living in the extreme depths are often small in size but can be very scary!

• Such pressure under the ocean!

Given that the average pressure at the surface of the sea is just over 1 bar, and that it increases by 1 bar every 10 m of depth, what is the pressure exerted in the deep trenches at -10,000 metres?

The correct answer is 1,000 bar, or more than one metric ton per cm², which is equivalent to the weight of a small car on a postage stamp!

• Abyssal menu: whale corpse on a bed of flurrying detritus

The amount of food available decreases the deeper you go. In the great depths, food is limited to a "flurry" of organic detritus falling down from the surface. Animals that live on the seabed filter the sand to find food scraps.

When a whale corpse sinks into the abyssal plain, hundreds of animals come to feed on this providential feast. To prepare for potential food shortages, scavenging animals build up energy reserves and slow down their metabolism to cope with periods of fasting.

• To begin with, it's cold... (and even colder at the bottom of the oceans)!

The deeper you go into the ocean, the more the water temperature falls, dropping to 2°C from a depth of 1,000 m to the deepest depths, regardless of the region or season. In 75% of ocean waters, the water temperature fluctuates between 0° and 6°C.

• It's hot in the abyss!

In some places in the abyss, there are ocean springs where the water has been heated by coming into contact with the magma that burns inside the earth, reaching temperatures of 400°C. When the water spurts out, the minerals in the water create hydrothermal vents that attract endemic species. A deep-sea oasis develops there!

In fact, around these hot springs, bacteria develop at temperatures of up to 115°C. The growth of the bacteria supports chemosynthesis: the bacteria transform the chemical compounds in the scalding waters into sugars that can be assimilated by the abyssal species and are necessary for their survival.

• Life in the abyss is vulnerable

The abyss is not spared from human activity. Overfishing of deep-sea species is a threat. Climate change, which is causing water temperatures to rise, is also affecting abyssal life.

• The abyss, a potential to be preserved!

The potential of the abyss, a little-known ecosystem, is enormous: biodiversity, molecules of therapeutic interest and mineral resources. Consequently, their exploitation must be reasoned and controlled in order to benefit everyone. The seabed and subsoil of the high seas, known as "The Zone", have been declared a Common Heritage of Mankind.

The International Seabed Authority, an autonomous intergovernmental body created under the aegis of the United Nations in 1994, monitors activities relating to seabed mineral resources in the international zone beyond the limits of national jurisdiction (exclusive economic zone).