On the west coast you may have noticed sea urchins, specifically purple ones (Strongylocentrotus purpuratus) are hidden in crevices in the rocks, some that seem not only the perfect shape, but so deep the urchins can’t escape! This is not a trick of your mind, but ‘intentional’. The purple sea urchin is one of several species of urchin that actively bore into rocks, both by abrading with their spines and grinding with their teeth.
Some species, or younger urchins leave their protective home at night to graze, returning home during the day. In some cases the urchin bores too deep, or grows to quickly and becomes wedged in its hole. I’ve seen that at Keeha beach, but no photo available. In that case it depends on the food to drift into the hole.
The purple sea urchin is quite famous for it’s boring, and has even been noted boring directly into steel pilings in California!
I found another boring urchin in Cuba, I believe it is Echinometer lucunter – the red rock urchin or rock boring urchin. Don’t take this identification too seriously, simply matching photos and biogeography is not the most accurate way to make a positive ID.