In the popular imagination, the seabed is a place as fascinating as it is frightening: dark realms populated by alien-like creatures, harbouring shipwrecks and secrets, and fuelling legends since the dawn of humanity. Unfortunately, however, the reality told to us by the depths of the Mediterranean is not one of mermaids or mythological creatures, but of rubbish. In what should be (and what we believed to be) one of the last unspoilt places on Earth, a group of researchers has found a sprawling carpet of waste. This is the dramatic finding of a mission carried out in 2020 in the Calypso deep, the deepest depression in the Mare Nostrum, published in the scientific journal Marine Pollution Bulletin. Down there, at a depth of 5,109 metres, the submarine Limiting Factor documented one of the highest levels of pollution ever recorded in an abyssal environment.

At the heart of the Mediterranean (and of the problem)

"The dive into the Calypso deep was linked to the Expedition deep ocean initiative, a global network of experts working on the sustainable management of deep ocean resources, with the aim of diving at the deepest sites in all oceanic and marine basins," explains Miquel Canals, one of the authors of the paper.

The construction of the submersible used to explore the abyss, along with its mother ship, was carried out by Victor Vescovo and his company Caladan Oceanic. Vescovo, "an explorer of both space and the oceans," was the first human being to have reached both the highest and lowest points on our planet, and on 10 February 2020, accompanied by Prince Albert II of Monaco, he led the expedition to the deepest point in the Mediterranean. Using state-of-the-art sensors, the expedition confirmed the exact depth of the Calypso deep, which had already been explored in 1965 by the legendary French bathyscaphe Archimède.

"As with all dives to very great depths, the main challenges were technical in nature," explains the University of Barcelona professor, "to ensure the safety of those involved and the ability to carry out the planned operations once on the seabed, as well as meteorological and logistical ones, including the descent, navigation along the bottom, and the ascent to the surface for recovery by the mother ship."

During the mission, researchers documented a total of 148 types of waste, 88 per cent of which consisted of plastic — in particular bags, industrial sacks, bottles, caps and lids. The density of marine litter (the technical term for waste found at sea) estimated from the observations, equal to 26,715 objects per square kilometre, is among the highest ever recorded in abyssal environments.

Waste detection in the Calypso deep © Marine Litter Bulletin

Images of the different types of waste identified on the seabed © Marine Litter Bulletin

Percentage and types of waste detected on the seabed © Marine Litter Bulletin

The Calypso deep, in fact, as the researchers explain, acts as a natural trap for plastic: surrounded by walls that plunge thousands of metres and subject to extremely weak currents, any object that sinks into it has very little chance of escaping. Canals highlights how most of the plastic originates from floating waste carried from the Adriatic and Ionian seas, driven by surface currents and then pulled down by natural processes such as biofouling. This term refers to the process by which microorganisms, algae or small marine animals attach themselves to surfaces submerged in water — such as plastic, metal or wood — increasing the weight of objects and thus accelerating their descent into the abyss.

"It is no longer a surprise": the voice of scientists

Unfortunately, finding waste on the seabed is no longer a surprise, but a sign of the times we live in — and once it reaches the bottom, waste can remain there for centuries.

Miquel Canals

The burial of human-made material takes place over timescales that vary greatly from object to object, depending on factors such as size and shape, as well as the material's capacity to deform and flatten under the weight of sediment accumulating on top of it. However, Canals offers a sense of scale: "For a quarter of a plastic bottle lying on the seabed, we could estimate a timescale of at least centuries."

The removal of waste is always "a delicate matter, because it could cause more harm than good to the habitat involved." For this reason, any potential removal operations must be carefully assessed on a case-by-case basis, and at present they are extremely costly. "For the time being," he concludes, "removing waste from the deep sea is not an option."

EU Joint Research Centre @EU_JRC

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Marine litter in the deep Mediterranean seabed is among the worst worldwide, our latest study reveals.

The study was conducted using a deep-sea submersible vehicle, by Caladan Oceanic, which captured footage of the seafloor.

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The Mediterranean is no stranger to these records (and it's not alone)

Canals's limited surprise stems from the fact that the presence of anthropogenic materials in the world's bodies of water is nothing new. From the canyons of Portugal to the South China Sea, high concentrations of plastic waste in deep environments are becoming increasingly common. Nevertheless, the values recorded at Calypso Deep are second only to certain Asian hotspots and far exceed those found in the Pacific and Atlantic oceans. In other parts of the Mediterranean Sea too, previous research has documented high levels of litter in submarine canyons or beneath historic shipping lanes, but never at such depths.

Plastic is light, mobile and persistent. The Mediterranean, enclosed and heavily impacted by human activity, traps and concentrates it.

Miquel Canals

A wake-up call to global leaders from the planet's abyss

For the authors of the research, this finding must serve as a wake-up call for policymakers, since the impossibility of removing waste that has accumulated at such depths makes prevention the only viable path forward. To achieve this, existing instruments such as the Marine Strategy Framework Directive and the EU Plastics Strategy, or the Mediterranean Action Plan, must be strengthened and integrated; but above all, the problem must be tackled at a broad scale, accounting for waste entering the sea both from land and from vessels, monitoring the seabed using harmonised methods, and reliably assessing the effectiveness of the measures adopted. In this context, the new international regulatory instruments on the horizon – such as the UN Global Plastics Treaty and the High Seas Treaty on Marine Biodiversity in areas beyond national jurisdiction – represent an unprecedented opportunity.

The challenge will be to advance the complex machinery of diplomacy while giving value to what lies beyond our sight, such as the ocean depths. For if human impact reaches the deepest point of the Mediterranean, there is no place too remote to be excluded from protection policies.