"Liminality" is described very much like a philosophical "rite of passage:"
...being on the "threshold" of or between two different existential planes...
In Coleridge's The Rime of the Ancient Mariner, this concept can be seen in several ways. It is important to remember that as a Romantic writer, Coleridge stressed (among other things in this epic poem) a return to nature—having a respect for nature.
The mariner who narrates the story of his strange, heart-wrenching and life-altering experiences at sea stands several times on a "threshold" between one "plane" and another. We see it when he chooses to sacrifice the peace and happiness he has known when he kills the albatross; when his fate is in the balance, pulled between life and death; and, later as he moves from a place of ignorance and disregard for nature, to an enlightened existence that honors nature, though he still suffers for his lack of respect earlier.
The mariner tells of sailing on a ship which is accompanied by an albatross—a large sea bird.
At length did cross an Albatross:
Thorough the fog it came;
As if it had been a Christian soul,
We hailed it in God's name. (62-65)
But one day, the mariner shoots the bird with his bow—for no good reason. This changes his luck and that of all the members of the crew:
...With my cross-bow
I shot the Albatross. (80-81)
Soon the weather changes, and conditions on the ship become dire, especially due to a lack of water—hence the following famous lines:
Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink. (116-119)
It is at this point that the ship's crew punishes the mariner by hanging the dead albatross around his neck. As things grow worse, the men see a sail, but it is a ghost ship. (This represents an element of the supernatural in the story.) It is here that Death and Life-in-Death throw dice to see who will win the souls of which sailors. Life-in-Death "wins" the mariner. This is another threshold on which the mariner stands—between living and dying. While the others die, he is spared.
The Night-Mare LIFE-IN-DEATH was she,
Who thicks man's blood with cold.
The naked hulk alongside came,
And the twain were casting dice;
“The game is done! I've won! I've won!” (190-194)
The mariner stands on the brink of the last threshold for some time. He watches as each of the two hundred other sailors drops dead on the deck, including his own nephew. The mariner is becalmed on the ship—with no water—and with all his dead shipmates. However, the mariner begins to watch the creatures in the sea, and in the moment he blesses them, he is able to pray; the bird falls from around his neck; the winds pick up; and, it begins to rain.
Beyond the shadow of the ship...
I watched the water-snakes… (269-270)
O happy living things! no tongue
Their beauty might declare:
A spring of love gushed from my heart,
And I blessed them unaware…
The selfsame moment I could pray... (279-285)
Though the mariner is eventually saved (when beings of light—angels?—inhabit the dead sailors and they all sail the ship home), the mariner's punishment continues—even as he has crossed this last "threshold" into an awareness of the beauty of nature: for as long as he lives, whenever he meets someone who is as he once was, his heart "burns" in his chest until he call tell his story, to change the heart of another.