Mysterious 'Neptune balls' washing up on Mediterranean beaches identified as seagrass clumps

Scientists say the rounded objects that have baffled beachgoers along Mediterranean shores are not alien artifacts or invasive debris but compact bundles of an endemic seagrass.

Locals, tourists and researchers had reported finding almost perfectly spherical or rugby ball–shaped objects — nicknamed "Neptune balls" — strewn across beaches after storm events. At a glance the balls can resemble fallen coconuts or curled-up animals and have drawn curiosity and speculation about their origin.

A Spanish team of researchers says the objects are made of Posidonia oceanica, a seagrass species native to the Mediterranean Sea. The researchers concluded that strong storms dislodge leaves and fibrous plant material from seabed meadows, and subsequent wave and current action rolls the fragments together until they form dense, rounded masses that eventually wash ashore.

The phenomenon follows a familiar pattern: strandings increase after periods of rough weather. Unlike the broad, flattened wrack lines of dead seagrass sometimes seen on beaches — known locally as banquettes — the Neptune balls are compact, three-dimensional aggregates. Similar rounded accumulations of seagrass fiber have been documented elsewhere and are known in scientific literature by terms such as egagropili.

Posidonia oceanica forms extensive underwater meadows that are a key component of Mediterranean coastal ecosystems. The seagrass helps stabilize seabeds, provides habitat and nursery grounds for marine life, and stores organic carbon in its biomass and sediments. Detached fragments and the resulting aggregates are a natural part of the plant's life cycle and of meadow dynamics, scientists say.

Researchers cautioned that while the objects are visually striking, they are not a sign of pollution and do not represent a novel synthetic material. Their appearance on beaches can nonetheless draw attention and occasional concern from the public. Scientists and conservationists say distinguishing natural wrack and aggregates from anthropogenic debris is important for both coastal management and public information.

The Spanish team's identification offers a straightforward explanation for the recurring strandings but also highlights links between weather patterns and coastal processes. Storm frequency and intensity influence how much seagrass is dislodged and how material is transported and compacted by waves and currents. Monitoring Posidonia meadows and beach strandings can therefore provide indirect information about the health of these habitats and the physical forces acting on them.

Researchers continue to document the distribution, size and composition of the Neptune balls to better understand formation mechanisms and any potential ecological effects once they reach shore. For now, the most consistent finding is that the objects are natural bundles of Posidonia fiber produced by the mechanical action of the sea, arriving on beaches in the wake of storms that reshape the Mediterranean coastline.