This poem gets a little spooky on us from time to time. We see different versions of the underworld throughout the poem: the dark and mysterious underwater underworld of the old aquarium, the underground garage under construction, graveyards, and the ditch where the dead soldiers were buried. Although the underworld imagery shifts shape, it is always hinting at the threat of a darker and more ominous world.
Lines 9–10: Here the speaker is referring to the days when he used to visit the aquarium. Though this seems to be a fond memory, there are explicit signs of a kind of underworld (specifically the "dark, downward […[ kingdom"). This is our first taste of something operating below the surface in this poem.
Lines 14–16: These lines are as straightforward as Lowell gets about the underworld. He imagines this underground parking garage as part of an underworld, which is a spooky and unwelcoming place.
Lines 43–44: The poem makes a quick pit stop in the place we're most accustomed to thinking about the underworld: a graveyard. Though we don't linger there long, it adds another layer to the underworld image that Lowell has been building throughout the poem.
Lines 50–51: Here the underworld reappears as a kind of grave. Of course, this isn't a formal grave. It's a crude and sad one, which is totally fitting as an example of the underworld image.
Line 53: We're pretty sure this isn't the same ditch that was mentioned in the previous stanza. This is more of a figurative, "big picture" ditch that leads us straight to the underworld. Yikes! Mind the gap.