The sound of horses' hoof-beats striking the ground has been described in poems and songs for millennia. One of the most famous onomatopoeic lines in Virgil's Aeneid is such a description:

quadrupedante putrem sonitu quatit ungula campum.

With such skillful descriptions as these abounding in Western (and, indeed, world) literature, it is very difficult to come up with a new and surprising way of describing a horse's gallop.

The "plunging hoofs" in the last line of "The Listeners" presents the reader with the poem's final image. A conventional description of hoof-beats on a road would clearly be an anti-climax. Something unusual is required to compel our attention.

The horse has been a stationary, rather placid presence throughout the poem, breaking the silence by eating grass noisily. The image of plunging hoofs contrasts with this, suggesting not just speed but extreme haste and barely controlled desperation. We associate plunging with water, which makes the word a surprising one to use of the hard dry "sound of iron on stone" mentioned two lines earlier. The first conventional image might be regarded as setting up and contrasting with the second unconventional one. This final image refers back to the mysterious atmosphere of the poem as a whole by allowing the reader to picture horse and rider rushing headlong into the unknown.