The bottom of the Calypso Deep, a 5,112m deep depression in the Ionian Sea, contains one of the highest concentrations of marine debris in the deep ocean: it is yet another testimony to the increasingly worrying human footprint that has turned seas and oceans into a vast waste dump. Credit: Caladan Oceanic. CC BY.

The bottom of the Calypso Deep in the Ionian Sea has one of the highest concentrations of marine debris in the deep ocean.

Waste generated by human activities has now reached the deepest point in the Mediterranean: the 5,112m-deep Calypso Deep in the Ionian Sea.

A total of 167 objects — mainly plastics, glass, metal and paper — have been identified at the bottom, of which 148 are marine debris and 19 others are of possible anthropogenic origin.

These results represent one of the highest concentrations of marine litter ever detected at great depths, according to the international team of researchers who found the debris.

To reach the bottom of the trench — the most critical aspect of the entire study — the team used a high-tech manned submarine, the Limiting Factor, a deep-submergence vehicle (DSV).

The images provided by the Limiting Factor confirm that, in addition to accumulating on coasts, surface waters and shallower bottoms, marine litter also reaches the deepest and most remote points of the Mediterranean, a sea that is particularly affected by human activities.

The debris identified in the Calypso Deep warns of the need to implement global policy actions to reduce ocean waste, as well as to encourage changes in citizens’ consumption habits and waste reduction in order to protect seas and oceans around the world.

The Limiting Factor submarine is capable of carrying two passengers to the deepest ocean trenches. Credit: Caladan Oceanic. CC BY.

Marine debris in the deepest Mediterranean Sea

The Calypso Deep is a depression located 60km west of the Peloponnese coast in Greece, within the so-called Hellenic Trench, with several similar but shallower depressions.

Located in an area of high seismicity due to active faults, it is surrounded by fairly steep, stepped relief and has slopes of thousands of metres, and a virtually flat bottom.

The inner part of the trench, more than 5,000m deep, is kidney-shaped and measures approximately 20km by 5km.

The debris at the bottom of the Calypso Deep comes from various sources, both terrestrial and marine. Credit: Caladan Oceanic. CC BY.

But how did the rubbish get so deep?

Miquel Canals, professor at the Department of Earth and Ocean Dynamics and director of the University of Barcelona Chair on Sustainable Blue Economy said the debris at the bottom of the Calypso Deep: “comes from various sources, both terrestrial and marine. It could have arrived by various routes, including both long-distance transport by ocean currents and direct dumping.

“Some light waste, such as plastics, comes from the coast, from where it escapes to the Calypso Deep, just 60km. Some plastics, such as bags, drift just above the bottom until they are partially or completely buried, or disintegrate into smaller fragments.”

He added: “We have also found evidence of the boats’ dumping of bags full of rubbish, as revealed by the pile-up of different types of waste followed by an almost rectilinear furrow.

“Unfortunately, as far as the Mediterranean is concerned, it would not be wrong to say that not a single inch of it is clean.”

The Calypso Trench traps and accumulates the anthropogenic materials that reach it at the bottom: “It is a closed depression, which favours the accumulation of debris inside it.

“The weak currents in the trench — about two centimetres per second and, exceptionally, 18 — also facilitate the deposition of light debris at the bottom.”

The currents carry floating debris mainly from the southern Ionian Sea and from marine areas further south. Surface eddies also tend to form, concentrating the debris inland.

When these eddies are located over the Calypso Trench, some debris tends to fall slowly to the bottom, aided by degradation mechanisms and ballasting processes that increase its density.

Surface currents can also transport debris from the Adriatic Sea to the north, through the Strait of Otranto, and from the waters off north-western Greece.

One of the most polluted

The first evidence of debris on the world’s seabed dates back to 1975, in the Skagerrak Strait in the North Atlantic.

Canyons and seamounts are the major accumulation points for debris, which can be buried, crumbled or moved by gravity, ocean currents and other oceanographic factors.

Today, the Mediterranean is a marine region particularly affected by this environmental problem; in 2021, a study already identified the Strait of Messina as the area with the highest known density of marine litter in the world, a real hotspot where large quantities of waste accumulate.

Canals said: “The Mediterranean is an enclosed sea, surrounded by humanity, with intense maritime traffic and widespread fishing activity.

“The evidence provided by our research should shake up global efforts, and in particular in the Mediterranean, to mitigate waste dumping, especially plastics, in the natural environment and ultimately in the sea, in line with the UN Global Plastics Treaty against plastic pollution, which is still pending approval.

“Even the papal encyclical Laudato si’ of May 2015 goes in this direction, as well as some declarations of the G7 world summits, although we are still far from achieving a great social echo in terms of seabed litter.”

Unlike other popular places, such as beaches or the coastline, “the ocean floor is still largely unknown to society as a whole, which makes it difficult to raise social and political awareness about the conservation of these spaces,” Canals remarked.

He added: “It is necessary to make a joint effort between scientists, communicators, journalists, the media, influencers and other people with social impact.

“The problem is there, and it has an enormous scope, even if it is not directly visible. We should not forget about it.”

The findings are published in Marine Pollution Bulletin.