Plastic in the deep sea: what we've found so far
The deep sea — below 200 meters, where sunlight doesn't reach — was the last place anyone expected to find plastic pollution. We were wrong.
The Mariana Trench plastic bag
In May 2018, an exploration of the Mariana Trench — the deepest known point in the ocean at 10,994 meters below sea level — captured video footage of a single-use plastic bag drifting at 10,898 meters depth. The image, published in Marine Policy, became one of the defining visual symbols of how thoroughly plastic has reached every region of the ocean.
Followup expeditions have found additional plastic items at hadal (ultra-deep) depths: bags, candy wrappers, fishing gear, and microplastic.
How does plastic get to the deep sea?
Three pathways:
- Sinking. Many plastics are denser than seawater (PVC, PET) and sink upon entering the ocean.
- Marine snow. Microplastic and organic particles aggregate and settle, carrying microplastic to the seafloor.
- Animal-mediated transport. Marine animals ingest plastic at the surface and excrete or die at depth, carrying plastic with them.
The result: deep-sea sediments are now considered the largest reservoir of microplastic on the planet, exceeding even ocean-surface concentrations.
Microplastic in the abyss
A 2020 study published in Science sampled deep-sea sediment from a 2,500-meter Mediterranean site. The finding: up to 1.9 million microplastic pieces per square meter of seafloor — the highest concentration of microplastic ever measured anywhere on earth. Concentrations were highest in seafloor topographic depressions where currents deposit sediment.
Animals at depth
Deep-sea ecosystems are slow-growing, low-energy, and especially vulnerable to disturbance. Studies have documented:
- Microplastic in the digestive tracts of deep-sea amphipods (small crustaceans) at depths exceeding 6,000 meters
- Plastic fibers in deep-sea coral tissue
- Plastic accumulation in deep-sea fish, including species with very small populations
Because deep-sea animals reproduce slowly and ecosystems regenerate over centuries, even modest plastic-related stress can have lasting consequences.
What this changes
The deep-sea findings change the strategic picture for ocean plastic policy:
- Cleanup is essentially impossible at depth. Surface-cleanup operations like The Ocean Cleanup can't reach sub-surface plastic, let alone the deep ocean.
- The ocean has no "safe sink." Plastic doesn't disappear by sinking — it accumulates in places that are harder to study and impossible to remediate.
- Source reduction is the only scalable solution. Once plastic reaches the deep sea, it's effectively permanent on human timescales.