Thousands of strange sponge-like yellow balls are washing up on beaches along France's western coast - and they've left holidaymakers and experts a bit baffled.
The mysterious objects have turned up across 18 miles of beaches along the Opal Coast, from Boulogne-sur-Mer to Le Touquet, according to a report in Atlas Obscura.
Many of these beaches, like La Slack, Wimereux, Le Portel, Equihen-Plage, Hardelot, Le Touquet, Stella, and Berck, are popular with tourists and authorities are attempting to clean up the clumps.
It is believed the balls are not pieces of marine sponge. A couple of explanations have been put forward.
The president of a French environmental organisation said the sponges smell like an "oil product" and could be remains of some sort of building material.
According to the Local, Jonathan Hénicart, president of the Sea-mer association, an NGO concerned with protecting the coasts, told French TV news channel BFMTV: "It seems to come from an oil product. It could come from a polyurethane product commonly used for building. And it smells very, very lightly of parrafin."
Over on Gizmodo, there were suggestions the balls could be the result of some kind of minor environmental disaster; possible sea foam created from when water and air are bound together by some kind of 'surfactant', a manmade substance in the water like detergent or fertiliser.
French authorities said the substance isn't a danger to public health, however as they don't know exactly what it is it may be best to be cautious.