The mysterious balls, which are not pieces of marine sponges, have disturbed locals and holidaymakers alike. Even though officials insist that they are not toxic or dangerous, people are advised to be careful not to touch them.
The French authorities aren't sure what the balls are, but researchers are investigating to find out. The sponges are said to smell like an oil product and some experts believe they could be the scattered remains of some kind of building material.
"It seems to come from an oil product," Jonathan Henicart, president of an NGO concerned with protecting the coasts told French TV news channel BFMTV.
"It could come from a polyurethane product commonly used for building. And it smells very, very lightly of paraffin," he added.
Meanwhile, a cleanup is already underway along the 30-kilometer stretch of the sandy beaches of northern France, overrun by the mysterious creatures of the deep people have never seen before.