I'm thinking this is the toughest sea creature mystery ever. This is a very important part of the life of a sea creature that is common along the northeastern Pacific shore. People who first see this on the beach think it looks like a plunger. It is smooth and rubbery to the touch. It is never completely round, always a partly completed circle. It is about as big around as, maybe two CDs. I found this one on a pebble beach, but they are usually found in sandy beaches.