We usually say that eyes adjust to the dark. Dave Bonta changes the phrase to "our ears adjust," and the whole poem follows from that swap. In a mine there is little to see, so hearing takes over, and "after some time" suggests patience: this is not a quick visit but a slow acclimatisation. The poem's central idea is that going underground is also going inward. The mine becomes a way of describing the body, and the body a way of describing death.
The second couplet makes that shift. "Hear back to the marrow / bone where we now go" can be read two ways. "To the marrow" is the familiar idiom for reaching the deepest part of something, as in being chilled to the marrow. But the line break lets "marrow" fuse with "bone" into "marrow bone," a thing rather than a figure of speech. Hearing travels inward through the skeleton until the listener is in the bone itself, "where we now go." The word "back" is also doubled. It can mean backward in space, down toward the core, or backward in time, toward whatever is oldest in us. The small word "now" matters too. It makes the descent feel like something happening as we read, and it hints that the place we are going is where we all end up, as bones.
The last couplet gives the poem its strangest image: "a disconsolate city / standing in amber." The grammar is loose enough that the city may be something heard rather than seen, since it follows the verb "hear." That fits a poem about listening in the dark. "Disconsolate" is a heavy word, meaning more than sad, close to beyond comfort. It casts the underworld as a place of mourning. Amber then freezes it. Amber is fossilised resin, famous for trapping living things in a moment that never ends, so the city is preserved and stuck at once. The colour also suggests the dim lamplight of a mine. Whether the city is a mythic realm of the dead, a ghostly settlement left underground, or the skeleton as a kind of architecture, the poem doesn't say, and the openness is deliberate.
The form carries as much meaning as the images. There are three unrhymed couplets, with no capital letters and no punctuation at all. Without full stops, the syntax slides from one line into the next, and the reader has to decide where each thought ends. That lets "hear back to the marrow" be a command, a statement or half of a compound word, depending on how you take it. The lines are short and uneven, from four syllables up to seven, so the poem seems to be feeling its way, like someone in a tunnel.
Sound does quiet work where rhyme is absent. The m of "mine," "marrow" and "amber" recurs across the poem like a low hum. "Bone where we now go" is almost entirely heavy monosyllables, and that slows the line to a walking pace, each word a footstep. Its long o sounds in "bone," "now" and "go" also give it a hollow, resonant quality, as if the line were echoing in a chamber. By contrast, "a disconsolate city" spreads over seven syllables, with its soft s sounds sounding like a whisper. Then "standing in amber" closes with a light, unstressed ending that stops rather than resolves. The poem ends in suspension, which suits its image of something held in resin.
The poem is significant for how much it does in six lines. It takes a physical fact, the way senses sharpen in darkness, and turns it into a small journey through earth, body and death without once explaining itself. The title "Underworld" invites echoes of ancient descents to the land of the dead, but the poem is quieter and more intimate than a grand myth. Its underworld is something you could reach just by listening carefully enough. The poem also makes the reader take part, since to follow it you have to slow down and listen as the speaker does. Having trained your ear on a few spare words, you end up standing in the amber yourself.