I've been collecting images with this theme in mind for some time. I like collecting words too: cave, niche, recess, cranny, nook, slot, pigeonhole, cavity, trench, crater, depression, orifice, vent, chink, cavern, chasm, canyon, ravine, crevice, fissure, cleft, slit. As I list these words I can think of many ways of arranging them: by size, into a series of haiku, by shape.
My photos of holes and hollows are on a small scale, taken on the beach and in the bush near my home: more crevice than ravine; more depression than crater; more niche than alcove; more chink than canyon.
While I was accumulating hole-and-hollow images I came across this image on the Redbubble site to add to my collection. It's also a good chance to share the site that has given me a lot of visual and cogitations pleasure.