Mary F. C. Pratt's poem takes us downward, layer by layer, beneath a piece of ground. First comes running water, then blue clay, then the stone body of a great fish, and deepest of all, the shards of a fallen star. Each layer reaches further back in time, from something moving and alive to something fossilized to something that fell from the sky.
The last two lines turn from the earth to "their bones." The word "their" isn't explained, but the poem seems to suggest that people, or perhaps the living beings of this place, carry something of that ancient material in them. The speaker doesn't sound alarmed or boastful. The tone is quiet and matter-of-fact, almost like someone passing on old knowledge.
The final line, "only a few of them understand," adds a gentle sadness. Most people walk over the place without sensing what lies beneath. The mood is hushed, mysterious, and a little reverent, as if the ground itself were a secret worth listening to.