Human trash has found its way to the deepest parts of Earth's seas and oceans

Human litter has now infiltrated the deepest parts of Earth’s oceans and seas. Debris and trash produced by human activity was spotted at a depth of 5,112 meters (3.2 miles) in the Mediterranean Sea.

The site, which is the deepest point in the Mediterranean, is known as Calypso Deep, and is located off the south-western coast of Greece in the Ionian Sea.

In a new study, researchers have identified 167 objects at the bottom. Of these, 148 were clearly marine debris, while 19 were believed to be of human origin, though less definitively.

According to the authors, this represents one of the highest concentrations of litter documented at great depths in any sea.

Debris in the bottom of the sea

To investigate the Calypso Deep, scientists used a crewed submersible called the Limiting Factor, a deep-submergence vehicle capable of withstanding extreme pressure.

Images captured by the Limiting Factor confirm that besides accumulating on coasts, surface waters and shallower bottoms, marine waste also reaches the deepest, most remote points of the Mediterranean Sea. 

Given the heavy impact of human activities on this nearly enclosed sea, the discovery emphasizes the urgency of global actions to reduce marine debris and a need for lifestyle changes that cut waste and protect marine environments worldwide.

The Calypso Deep as a debris trap

Located about 60 kilometers (37 miles) south west of the Peloponnese coast in Greece, the Calypso Deep is a depression within the Hellenic Trench system.

Surrounded by steep terrain in an area with active faults, it has slopes measuring thousands of meters and a mostly flat floor at its lowest point.

Its deeper, kidney-shaped portion extends over approximately 20 by 5 kilometers (12 by 3 miles) at depths surpassing 5,000 meters (3 miles).

Litter’s journey to the bottom

But how does rubbish collect so far down?

According to Miquel Canals from the University of Barcelona’s Faculty of Earth Sciences, the debris 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. 

Canals explains that plastic items, for instance, can drift from coastal waters.

Some light waste, such as plastics, comes from the coast, from where it escapes to the Calypso Deep, just 60 kilometers [37 miles] away.

Some plastics, such as bags, drift just above the bottom until they are partially or completely buried, or disintegrate into smaller fragments.

“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,” Canals continued.

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

Debris transport across the sea

Canals noted that the Calypso Deep is a closed depression, which favors the accumulation of debris inside it. He pointed out that the weak currents in the trench also facilitate the deposition of light debris at the bottom.

Floating debris originates mostly in the southern Ionian Sea or further south, and drifting eddies bring it toward the Calypso Deep.

“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,” explained Canals

“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.”

Technology to explore the abyss

Investigating such extreme depths demands cutting-edge tools. In this case, the Limiting Factor submarine, built by Triton Submarines, allowed a small crew of two to descend.

Moving at about 1.8 kilometers (1.1 miles) per hour to obtain clear images, the Limiting Factor covered roughly 650 meters (0.4 miles) in a straight line over a 43-minute window on the bottom. 

“On each dive, more time is usually spent descending and ascending the vehicle back to the surface than inspecting the bottom. Each complete dive usually takes a few hours,” Canals noted.

The expedition revealed no substantial impact on local marine life, likely due to the area’s low biodiversity.

Only a few species – namely, the fish Coryphaenoides mediterraneus and the crustacean Acanthephyra eximia – appeared in the footage. 

“However, in places with more biodiversity, there are different types of interaction between the debris deposited on the bottom and the organisms, such as ghost fishing, burial, hooking or ingestion, but also the use of debris as a substrate where animals can grow, hide or lay their eggs,” Canals said.

Mediterranean’s pollution problem

Scientists first documented trash on the ocean floor in 1975, in the North Atlantic’s Skagerrak Strait. Areas like canyons and seamounts are especially prone to accumulating garbage, either from settling, burial, or erosion processes.

Now, the Mediterranean ranks among the worst-affected seas. In 2021, research identified the Strait of Messina as the global hot spot for seafloor litter.

This highlights broader concerns about intensifying human influence on an enclosed region subject to heavy maritime traffic and fishing.

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

“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.”

Canals emphasized that 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.

“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.”

Details of the study are published in the Marine Pollution Bulletin.

Image Credit: Caladan Oceanic

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