At a brief glance, they look like fallen coconuts, or even curled-up furry animals.
They could even be mistaken for carefully-arranged deposits from some kind of extraterrestrial civilization.
Whatever they are, these curious balls washing up across Mediterranean shores are certainly out of place.
Some are almost perfectly spherical, while others are shaped like rugby balls.
They've been dubbed 'Neptune balls', but they've got nothing to do with the eighth planet from the sun.
They're named after the Roman god of the sea who can control ferocious winds and storms.
Locals, tourists and even scientists have been left baffled by Neptune balls, which often are beached following storms.
But a Spanish team of researchers think they know what these objects are – and why exactly they keeping appearing.
Neptune balls are round, compact bundles of a seagrass species called Posidonia oceanica, which is found in the Mediterranean Sea.
A common name for Posidonia oceanica is Neptune grass, which is why they are given the name 'Neptune balls'.
Anchored to the seabed, the plant's leaves fall off and gather together like some kind of underwater tumbleweed, forming loose spheres.
These spheres collect plastic as they form, including bits of food packaging, carrier bags, twine, bottle caps and much more.
They finally wash up on the shore, where they act as a reminder to beach-dwellers of the scale of humanity's plastic obsession.
In this way, the ingenious plant is effectively removing the plastic from the sea and returning it to where it came from.
According to scientists, swaying Posidonia oceanica on the seafloor start becoming tangled up in plastic before their leaves have even detached.
And some of these are 'microplastics' – plastic particles less than 5 millimeters in diameter, invisible to the naked eye.
Gruesomely, Neptune balls can even contain sanitary towels, tampons and used wet wipes, after they've been flushed down the toilet.
'[These are] things with a lot of cellulose, so they sink,' Professor Anna Sànchez-Vidal, earth sciences lecturer at the University of Barcelona, told the BBC.
In 2021, Professor Sànchez-Vidal published a study in the journal Scientific Reports that addressed the scale of plastic pollution in Posidonia oceanica, which is a species found only in the Mediterranean Sea.
In 2018 and 2019, they had counted the number of plastic particles found in balls that had washed up on four beaches in Mallorca, Spain, which has large seagrass meadows offshore.
They found there was plastic debris in half of the loose seagrass leaf samples – up to 600 bits per kilo of leaves.
Only 17 per cent of the tighter bundled seagrass fibre contained plastic, but at a much higher density – nearly 1,500 pieces per kilo of seaball.
The shape and arrangement of the seagrass underwater makes it an effective filter and trap for plastics in the coastal areas.
Human debris can enter waters after being left at the beach, blown by wind, or being flushed down the toilet and escaping from nearby sewage works.
There are some 70 species of marine seagrass, which evolved roughly 80 million to 100 million years ago from grass on land.
Growing from the Arctic to the tropics, most species have long, grass-like leaves that can form vast underwater meadows.
But they are not just pretty, playing a role in improving water quality by absorbing excess nutrients and trapping sediment.
They are also weapons in the climate crisis as they absorb CO2 and exude oxygen, and are a natural nursery and refuge for hundreds of species of fish.
By anchoring shallow waters, they also help prevent beach erosion, and dampen the impact of destructive storm surges.
