Just a rock (actually, more than one) exposed at low tide at Cabrillo National Monument, California, USA. Erosion has gouged out little smooth holes in the rock in the center, and in others in the area. Nature seems to abhor a vacuum -- most of the holes are filled by other rocks, which, I suppose, will, with the action of the waves, erode the holes further.
This rock in the center with all the holes is roughly fist-size. Larger sizes of this photo are available. (Click on the magnifier icon above the photo)
June 2013: Thanks, viewers! This photo has been viewed over 15,000 times, and is in thirteen galleries, both of which amaze me!