Marine Debris Hidden Inside Underwater Caves

There is a persistent illusion we carry about the ocean—that it is vast enough to absorb everything we throw into it. If plastic disappears from the beach, we assume it has dissolved into the blue infinity, gone for good. Out of sight, out of mind. The truth is far less forgiving.

Along rugged coastlines, behind limestone cliffs and narrow rock openings barely accessible by small boats or experienced divers, the sea keeps its own record. Inside sea caves—dark, confined spaces shaped by waves over thousands of years—marine debris does not vanish. It accumulates, settles, and stays. These caves, once considered refuges for marine life, are increasingly becoming hidden storage sites for human waste.

A recent scientific study reveals that sea caves are not passive victims of pollution, but efficient traps, silently collecting plastic and other debris that the ocean can no longer release.

This alarming reality is documented in a study published in Marine Pollution Bulletin in February 2026, titled The Untold Story of Sea Caves: Lost and Found Anthropogenic Debris in Confined Marine Environments. The research was led by Ioannis Savva, alongside Leda L. Cai, Melina Marcou, and D. L. Angel, a team of marine scientists specializing in coastal and benthic ecosystems.

Their research focused on semi-submerged sea caves in the Mediterranean region—environments often excluded from standard marine pollution surveys due to their inaccessibility. What they found was unequivocal: every surveyed sea cave contained anthropogenic debris.

“We did not encounter a single sea cave that was free from human-made waste,” Savva notes in the study. Debris density ranged dramatically, from 0.044 to 96.839 items per square meter, with some caves exhibiting levels comparable to—or even exceeding—those found on open beaches and the seafloor.

These findings challenge a long-held assumption that confined marine environments are naturally protected from pollution.

Plastic Dominates

Approximately 66 percent of the debris identified was plastic, including packaging fragments, synthetic ropes, and degraded consumer waste. Many of these items were heavily weathered, suggesting long-term entrapment rather than recent deposition.

According to Leda L. Cai, plastic’s physical properties make it especially prone to becoming trapped in caves. “Plastic is light and easily transported by waves and currents,” she explains. “But once it enters confined spaces like sea caves, the hydrodynamics prevent it from escaping.”

One of the study’s most striking discoveries was the presence of “plastitar”—plastic items embedded within hardened tar layers coating cave walls and floors. This phenomenon indicates that pollution is no longer just contaminating ecosystems; it is physically altering geological structures.

In these caves, plastic is no longer a foreign object—it becomes part of the environment itself.

Melina Marcou, who analyzed the spatial distribution of debris, emphasizes the role of cave morphology. Long corridors, narrow entrances, and limited water circulation create ideal conditions for debris retention.

“Caves with small internal beaches and elongated chambers consistently showed higher debris accumulation,” Marcou explains. “Waves push items inward, but the energy required to remove them simply isn’t there.”

As a result, debris migrates toward the deepest and darkest sections of caves—areas once considered ecological sanctuaries. This process transforms sea caves into one-way systems for waste, where human debris enters easily but almost never leaves.

Silent Threats to Marine Life

The ecological consequences extend far beyond visual pollution. Trapped debris releases chemical additives, hosts pathogenic microorganisms, and gradually breaks down into microplastics. In confined cave environments, these processes may intensify due to limited water exchange.

D. L. Angel, an expert in marine ecology and co-author of the study, warns of long-term biological risks, particularly for species that depend on sea caves for survival.

“Sea caves are critical habitats for a range of organisms, including endangered species,” Angel says. “When debris accumulates in these confined spaces, the ecological risks increase exponentially.”

One such species is the Mediterranean monk seal (Monachus monachus), one of the world’s most endangered marine mammals. Monk seals rely on sea caves for resting, breeding, and nursing their pups.

Ironically, the most protected sections of caves—the deepest and least disturbed—are often the most polluted.

While direct mortality from macro-debris has not yet been documented in these caves, the chronic exposure to microplastics and chemical contaminants poses a serious long-term threat.

A Global Problem

Although this study focuses on the Mediterranean, its implications are global. Sea caves exist along coastlines worldwide, from southern Europe to Africa, the Americas, and Southeast Asia. Yet they remain largely absent from marine pollution monitoring programs.

Savva stresses that the absence of data does not indicate the absence of pollution. “Any coastal region with sea caves is potentially facing the same issue,” he notes. “We simply haven’t been looking.”

Because of their isolation, sea caves act as long-term archives of marine pollution, storing debris for decades and offering a historical record of human impact on the ocean.

Indonesia represents a particularly vulnerable case. As the world’s largest archipelagic nation, it has extensive karst coastlines and countless sea caves and submerged cave systems. Yet systematic research on debris accumulation in Indonesian sea caves is virtually nonexistent.

Indonesia is already recognized as one of the largest contributors to marine plastic pollution globally. Plastic has been documented on beaches, coral reefs, the deep sea, and even ocean trenches. Given this reality, it is highly unlikely that Indonesian sea caves remain unaffected.

The country’s complex ocean currents—linking the Pacific and Indian Oceans—may even accelerate debris transport into coastal cave systems. Many of these caves are difficult to access, meaning accumulated waste may remain undetected for generations.

In this context, sea caves may represent one of Indonesia’s most overlooked environmental frontlines.

This research sends a clear message, that the ocean does not erase our waste. It redistributes it, hides it, and preserves it in places we rarely see. Sea caves remind us that marine pollution is not always floating on the surface or stranded on beaches. Sometimes it hides in darkness, slowly transforming ecosystems that once offered refuge to life.

If current patterns of consumption and disposal continue, future divers may enter sea caves not to admire their geological beauty, but to witness the fossilized remains of our plastic age. The sea will not accuse us. It has already kept the evidence—quietly, patiently, in the dark. (Wage Erlangga)