Charm'd magic casements, opening on the foam Of perilous seas, in faery lands forlorn. (lines 69-70)
Near the end of the poem, the speaker gets kind of carried away and leaves normal reality behind altogether. The natural world is no longer natural – it has become sheer fantasy. He imagines the nightingale flying out of magic windows in another fairy kingdom. His imagination is kicked into overdrive, and this is a sign that he's about to have a rude awakening.
Past the near meadows, over the still stream, Up the hill-side; and now 'tis buried deep In the next valley-glades: (lines 76-78)
Like the magic bottle of wine from line 13 that is buried in the earth, the nightingale "buries" itself in the next valley. It quite literally flies in the face of the speaker's attempts to turn nature into his own private fantasy. The distance between the speaker and nature is reestablished.