It's pretty clear that the world of Dream-Land looks nothing like any place we've ever been. Poe lets us know that we're in a really different world by tacking on words like "boundless" and "bottomless." These words are kind of descriptive – we can imagine what he's talking about, but they are also a little mysterious. We can't know exactly how deep or wide these oceans and valleys are. Poe wants to keep our vision a little hazy, a bit dream-like.
Mountains toppling evermore Into seas without a shore; (line 13-14)
Crazy stuff is happening in this landscape. Not only is everything really huge, but it's churning and exploding and collapsing all the time. In the real world, a mountain falling into the ocean would be a pretty big deal, but here it seems to happen all the time. This explosive action adds a little bit of excitement to the poem, makes us feel as if incredible things are happening – as if we were watching a movie with great special effects.
With the snows of the lolling lily. (line 20)
This is sort of a funny, quiet little image. It's not scary or overwhelming or gloomy like a lot of the other natural stuff in this poem. We imagine a pretty white field of snow. As for those lolling lilies, maybe Poe is using the image of a white lily to echo the whiteness of the snow, or maybe there are actually little flowers poking up. Either way, this is kind of a relaxing, happy natural image in a poem that is otherwise pretty intense.
By the mountains— near the river Murmuring lowly, murmuring ever, — (lines 25-6)
We think this is a good example of how this poem is tied into the rhythms of nature. Poe doesn't just happen to mention a river, he gives us a real feeling for how it sounds. In these lines, it almost becomes a character in the poem, speaking quietly to us as it rolls by. This isn't designed to make us scared or upset, instead it gives us a kind of quiet sense of the world we're exploring. It's like a little detour on this sad journey, a peaceful little bit of nature poetry that lets us shift gears for a second.
By the grey woods,— by the swamp Where the toad and the newt encamp— By the dismal tarns and pools (lines 27-9)
This is definitely not about the happy or beautiful side of nature. These lines are all about the icky, slithery, unpleasant side of the natural world. We've heard some beautiful descriptions in this poem, but now we're just getting pulled through the muck. Everything is grey and dirty and unhappy. Gross, we know, but this is a Poe poem after all, so there has to be some of that.