The poem's vivid imagery conveys the appearance and consistency (or inconsistency) of the treacherous "Bogland."
According to the speaker, the bog is "wet," full of "bogholes," and ultimately as "bottomless" as the sea, because its watery "seepage" comes from the "Atlantic" Ocean. It's so damp beneath the surface that it reduces "great firs" to "waterlogged trunks / [...] soft as pulp." However, the surface "keeps crusting" over in the daytime heat, making it walkable, if hazardous.
Though most of the poem's imagery appeals to the senses of sight and touch, some startling taste imagery enters lines 13-15:
Butter sunk under
More than a hundred years
Was recovered salty and white.
Because the cool bog acts as a preservative, pre-modern Irish communities buried food containers in it, using it as a crude form of refrigeration. Some of those containers have indeed been "recovered" over the years, and some have contained dairy (known as bog butter) that's remarkably well-preserved. The poem describes a specimen of this butter, aged "More than a hundred years," as "salty and white"—practically daring the reader to imagine taking a bite.
The poem then compares the boggy "ground" itself to butter. The adjectives "kind," "black," "Melting," and "opening" turn this metaphor into a vivid image. They indicate that the soil is soft (figuratively, "kind") and very dark, and that it slips around and caves in as you walk over it. (Notice, too, how the "white" literal butter makes for a striking visual contrast with the "black," figurative butter.)