Recently, on France’s western coast, thousands of sponge-like balls have been appearing, strewn across 18 miles of beaches, from Boulogne-sur-Mer to Le Touquet. And no one seems to know where they are coming from.

The balls, which are not pieces of marine sponges, have disturbed locals, though officials have said that they are not toxic or dangerous.

According to BMFTV, a clean-up is now underway. The president of a French environmental organization telling the broadcaster that sponges smell like an “oil product,” and could be the scattered remains of some kind of building material.

Alternatively, as Gizmodo points out, the sponges could be the result of some minor environmental disaster: Sea foam created when water and air are bound together by some kind of human-generated stuff in the waters off the coast—like a detergent of some sort, or raw sewage.

Whatever it is, it’ll surely ruin your day at la plage.