The mysterious shapes, which resemble alien eggs, were discovered by shocked beachgoers.
Witnesses said there were so many at one point it looked like a carpet across the sands.
Now experts are attempting to identify the creatures, which burrowed into the beach.
Scientists from the National Oceanic and Atmospheric Administration couldn’t identify the creatures from photos.
However they guessed it must be some form of burrowing sea cucumber, TV station NBC Los Angeles reported.
Matt Bracken of the University of California, Irvine, said they were likely “pelagic tunicates”.
Better known as salps, the creatures “occasionally bloom off the California coast” he told the OC Register.
“These marine invertebrates look sort of like jellyfish, but they are actually more closely related to humans,” he said.
Marine biologist Bruno Pernet of California State University in Longbeach said a recent storm had likely caused the creatures to wash up.