The poem begins with a question: how deep would we have to dig to find a place untouched by life? The answer it gives is unsettling. Even far underground, some bacteria live on the energy of radioactive decay, with no need for oxygen, sealed in their own tiny universe. Life, it turns out, is nearly everywhere, with about half of its total mass hidden below the surface or under the ocean floor.
From there the speaker turns wry and a little guilty. Life becomes an infection, and humans are its most obvious carriers. The planet is "the poor thing," stuck with us, while the moon, "a barren mate," stays clean and distant. The final image is the moon's tug on the tides, which we flatter ourselves by imagining is meant for us.
The mood is quietly ironic, mixing wonder at life's toughness with self-mockery about human vanity. It leaves you with the feeling that stillness may be something we can only look at from far away.