I have a problem.  I like to collect beach stones.  Many times I've found stones and rocks like this one with perfect holes bored right through them.  Today I found out that there isn't some dude sitting out in a boat carving holes out in his spare time.  These holes are actually created by piddocks, a bivalve mollusk similar to a clam.  One side of the shell has a set of ridges that they use to burrow inside of clay or soft rock. When they die, the mollusk will often wash away but you may find its shell....or just a clean burrow like this one.