What is a description of the underground world of the sea where the merman and his family resides?

In this poem by Matthew Arnold, the "kind sea-caves" where the merman and his kin live beneath the surface represent only one of two worlds depicted in the narrative. There is rather more description of the world above water, with its little grey church. What we are told about the underwater world, however, is that it is possible to hear from there the sound of "a far-off bell" which calls from the land above ground. The merman's lover heard this bell when she was sitting with him "on a red gold throne in the heart of the sea." Upon this throne, the woman sat with her beloved and combed the hair of her youngest child. The speaker of the poem now addresses these children as he speaks of what has happened to their mother.
The speaker describes the underwater world as being full of "sand-strewn caverns, cool and deep." It is a peaceful world, as we can see from words such as "asleep" and languid verbs such as "sways," which describe the actions of the underwater plants. Lights "quiver and gleam" in this world, but they are "spent," suggesting that they are not very strong, leaving the caverns generally cool and dark. There are other creatures in this world, too: "sea-beasts" who are described as being at pasture, as if they were animals domesticated on the ground above. Meanwhile, "sea-snakes" and "whales" pass through the area, but without any malice, "sailing" instead. The sound of the bell, then, comes as a disturbance into this peaceful, drifting world, calling away the children's mother and changing the lives of everyone in the sea-caves.