But why? Why does it have so much power? I think the answer is three-fold: description, character, and ambiguity.
First, description. I've always argued that Tolkien's command of language in the pursuit of imagery is all but unparalleled. Many of the passages describing the character's journey and the landscape they passed through leaped out to me as exhileratingly familiar, despite my fifteen-year absence from them. Despite the time that's passed, I remembered them and they resonated with me anew. Other passages were exciting in that they seemed completely new to me. Years of experience reading and writing allowed me to conjure up completely new images of setting and action. I think that is one of the most important jobs of a work of fiction -- it must transport the reader to a new place, must evoke new pictures and ideas. Take some of these passages:
"Indeed the whole surface of the plains of Gorgoroth was pocked with great holes, as if, while it was still a waste of soft mud, it had been smitten with a shower of bolts and huge slingstones. The largest of these holes were rimmed with ridges of broken rock, and broad fissures ran out from them in all directions. It was a land in which it would be possible to creep from hiding to hiding, unseen by all but the most watchful eyes: possible at least for one who was strong and had no need for speed. For the hungry and worn, who had far to go before life failed, it had an evil look."
"The confused and tumbled shoulders of [the mountain's] great base rose for maybe three thousand feet above the plain, and above them was reared half as high again its tall central cone, like a vast oast or chimney capped with a jagged crater. But already Sam was more than half way up the base, and the plain of Gorgoroth was dim below him, wrapped in fume and shadow.... [A]mid the rugged humps and shoulders above him he saw plainly a path or road. It climbed like a rising girdle from the west and wound snakelike about the Mountain, until before it went round out of view it reached the foot of the cone upon its eastern side."And finally:
"Fearfully he took a few uncertain steps in the dark, and then all at once there came a flash of red that leaped upward, and smote the high black roof. Then Sam saw that he was in a long cave or tunnel that bored into the Mountain’s smoking cone. But only a short way ahead its floor and the walls on either side were cloven by a great fissure, out of which the red glare came, now leaping up, now dying down into darkness; and all the while far below there was a rumour and a trouble as of great engines throbbing and labouring. The light sprang up again, and there on the brink of the chasm, at the very Crack of Doom, stood Frodo, black against the glare, tense, erect, but still as if he had been turned to stone."Objects anthropomorphized; subtle alliteration; stark imagery evoked by harsh adjectives. My command of sentence structure and my ability to articulate how the precise use of words generates a particular effect is at a low ebb; it's been almost a decade since I've performed any such analysis. But I know a master of the art when I see one.
Second, character. I won't belabor this point too much, since Jacob already addressed it to great effect, but Tolkien takes Frodo, Sam, and Gollum (who, along with Aragorn are the central figures of the narrative) to the peak of their development in the climax. What's wonderful is that these characters are changing, evolving, reacting in what feels like a real way, to the events of the plot up until the very last instant.
Sam is presented with the reality that there is no coming back from the Mountain, and his resolve hardens, rather than shatters, as a result. Later, he comes to grips with his relationship with Gollum, something he's grappled with and we as readers have criticized him for throughout the last few books. In the end, faced with the choice of whether to kill Gollum, Sam makes the decision to spare his life -- just as Frodo and Bilbo did before him -- and thus forges the latest chain in the link that results in the victory of good over evil. Frodo, acted upon by the power of the Ring from almost the first moments that he becomes its bearer, makes the choice to seize it for himself. This turn of events was presaged throughout the chapter, with Frodo uttering increasingly ominous phrases ("It is mine, I say"; "I am almost in its power now"; "all else fades"), and yet it is still a shock to hear him utter it with such finality in the depths of the Sammath Naur. And Gollum, that pitiable creature, is at his lowest-ever moment (the realization that his Master is here to destroy his Precious) as well as his highest-ever moment ("his eyes were lifted up to gloat on his prize, he stepped too far...") Readers speak of the success of texts in terms of how dynamic characters are written. These feel real. We know them. And yet we are surprised by them. Just like life.
Third and finally, ambiguity. More than ever, at the end of the chapter, I was left wondering just what Tolkien meant for us to take away from all of this. Is it an extended Christian allegory -- Frodo, the everyman, cannot complete the journey on his own and must be carried, and the burden lifted, by other parties? But that doesn't square with Frodo's utter failure in the face of the temptation and power of the Ring -- and the fact that Gollum is not an enabling figure, but one who takes away the source of temptation by force. Is it an illustration of Gollum as the ultimate addict -- his desire is his ultimate undoing? But that doesn't square with the realization that Gollum's addiction is the only thing that saved Middle-earth in that moment. What exactly are we to make of Frodo's failure? Is Tolkien saying that there is no such thing as someone who is truly heroic in the face of unspeakable evil? After all, Sam, now a Ring-bearer himself, acknowledges the unspeakable weight of that burden, with the implication that he himself might have fared no better than Gollum: "[N]ow dimly he guessed the agony of Gollum’s shrivelled mind and body, enslaved to that Ring, unable to find peace or relief ever in life again."
This ambiguity of meaning is found in the title of the chapter itself: "Mount Doom." The origin of the word "doom" is Germanic, where it meant "to put in place," signifying an inescapable outcome, sometimes with a legal connotation, as in the inescapable consequences of a broken law or poor choice. Was Frodo's decision to claim the Ring inescapable? Given what he went through during the last three books, it's hard to say it wasn't; his will was finally overcome. But at the same time, we have Sam's pivotal choice to spare Gollum, as others did before him -- the choice that saved the world. Was that outcome, too, inevitable?
So why is it, as I walk with Sam down into the mountain, that I wait with baited breath that maybe, maybe this time, Frodo will make the right choice, and throw the Ring into the fire? There has to be a choice, doesn't there?
Therein, perhaps, lies the power of the text. It's one that keeps you asking questions long after the chapter itself has been read. It forces you to confront the characters' choices and experiences in light of your own tendencies, strengths, and weaknesses. Something we all do far too infrequently.