Times ECHO is mentioned in the book: 

Adela Quested (Q) has a party and invites Indian people but it’s very awkward because the Indian people don’t feel comfortable and are trying to be accommodating and polite. Quested refers to “the echoing walls of their civility.”

“Miss Quested now had her desired opportunity; friendly Indians were before her, and she tried to make them talk, but she failed, she strove in vain against the echoing walls of their civility.”

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Mrs. Moore’s experience in the caves with Miss Quested and Aziz:

“A Marabar cave had been horrid as far as Mrs. Moore was concerned, for she had nearly fainted in it, and had some difficulty in preventing herself from saying so as soon as she got into the air again. It was natural enough: she had always suffered from faintness, and the cave had become too full, because all their retinue followed them. Crammed with villagers and servants, the circular chamber began to smell. She lost Aziz and Adela in the dark, didn’t know who touched her, couldn’t breathe, and some vile naked thing struck her face and settled on her mouth like a pad. She tried to regain the entrance tunnel, but an influx of villagers swept her back. She hit her head. For an instant she went mad, hitting and gasping like a fanatic. For not only did the crush and stench alarm her; there was also a terrifying echo.

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Professor Godbole had never mentioned an echo; it never impressed him, perhaps. There are some exquisite echoes in India; there is the whisper round the dome at Bijapur; there are the long, solid sentences that voyage through the air at Mandu, and return unbroken to their creator. The echo in a Marabar cave is not like these, it is entirely devoid of distinction. Whatever is said, the same monotonous noise replies, and quivers up and down the walls until it is absorbed into the roof. “Boum” is the sound as far as the human alphabet can express it, or “bou-oum,” or “ou-boum,”—utterly dull. Hope, politeness, the blowing of a nose, the squeak of a boot, all produce “boum.” Even the striking of a match starts a little worm coiling, which is too small to complete a circle but is eternally watchful. And if several people talk at once, an overlapping howling noise begins, echoes generate echoes, and the cave is stuffed with a snake composed of small snakes, which writhe independently.

After Mrs. Moore all the others poured out. She had given the signal for the reflux. Aziz and Adela both emerged smiling and she did not want him to think his treat was a failure, so smiled too. As each person emerged she looked for a villain, but none was there, and she realized that she had been among the mildest individuals, whose only desire was to honour her, and that the naked pad was a poor little baby, astride its mother’s hip. Nothing evil had been in the cave, but she had not enjoyed herself; no, she had not enjoyed herself, and she decided not to visit a second one.”

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 After Mrs. Moore sends Quested and Aziz on their way, she thinks about this soon after:

“No, she did not wish to repeat that experience. The more she thought over it, the more disagreeable and frightening it became. She minded it much more now than at the time. The crush and the smells she could forget, but the echo began in some indescribable way to undermine her hold on life. Coming at a moment when she chanced to be fatigued, it had managed to murmur, “Pathos, piety, courage—they exist, but are identical, and so is filth. Everything exists, nothing has value.” If one had spoken vileness in that place, or quoted lofty poetry, the comment would have been the same—“ou-boum.” If one had spoken with the tongues of angels and pleaded for all the unhappiness and misunderstanding in the world, past, present, and to come, for all the misery men must undergo whatever their opinion and position, and however much they dodge or bluff—it would amount to the same, the serpent would descend and return to the ceiling. Devils are of the North, and poems can be written about them, but no one could romanticize the Marabar because it robbed infinity and eternity of their vastness, the only quality that accommodates them to mankind.

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She tried to go on with her letter, reminding herself that she was only an elderly woman who had got up too early in the morning and journeyed too far, that the despair creeping over her was merely her despair, her personal weakness, and that even if she got a sunstroke and went mad the rest of the world would go on. But suddenly, at the edge of her mind, Religion appeared, poor little talkative Christianity, and she knew that all its divine words from “Let there be Light” to “It is finished” only amounted to “boum.” Then she was terrified over an area larger than usual; the universe, never comprehensible to her intellect, offered no repose to her soul, the mood of the last two months took definite form at last, and she realized that she didn’t want to write to her children, didn’t want to communicate with anyone, not even with God. She sat motionless with horror, and, when old Mohammed Latif came up to her, thought he would notice a difference. For a time she thought, “I am going to be ill,” to comfort herself, then she surrendered to the vision. She lost all interest, even in Aziz, and the affectionate and sincere words that she had spoken to him seemed no longer hers but the air’s.”

 

Later, Adela Quested is in another English person’s home and reflects on the echo, as well as wishing that Mrs. Moore would visit her:

“Mrs. Moore, the only visitor she wanted, kept away. No one understood her trouble, or knew why she vibrated between hard commonsense and hysteria. She would begin a speech as if nothing particular had happened. “I went into this detestable cave,” she would say dryly, “and I remember scratching the wall with my finger-nail, to start the usual echo, and then as I was saying there was this shadow, or sort of shadow, down the entrance tunnel, bottling me up. It seemed like an age, but I suppose the whole thing can’t have lasted thirty seconds really. I hit at him with the glasses, he pulled me round the cave by the strap, it broke, I escaped, that’s all. He never actually touched me once. It all seems such nonsense.” Then her eyes would fill with tears. “Naturally I’m upset, but I shall get over it.” And then she would break down entirely, and the women would feel she was one of themselves and cry too, and men in the next room murmur: “Good God, good God!” No one realized that she thought tears vile, a degradation more subtle than anything endured in the Marabar, a negation of her advanced outlook and the natural honesty of her mind. Adela was always trying to “think the incident out,” always reminding herself that no harm had been done. There was “the shock,” but what is that? For a time her own logic would convince her, then she would hear the echo again, weep, declare she was unworthy of Ronny, and hope her assailant would get the maximum penalty. After one of these bouts, she longed to go out into the bazaars and ask pardon from everyone she met, for she felt in some vague way that she was leaving the world worse than she found it. She felt that it was her crime, until the intellect, reawakening, pointed out to her that she was inaccurate here, and set her again upon her sterile round.

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If only she could have seen Mrs. Moore! The old lady had not been well either, and was disinclined to come out, Ronny reported. And consequently the echo flourished, raging up and down like a nerve in the faculty of her hearing, and the noise in the cave, so unimportant intellectually, was prolonged over the surface of her life.”

 

Finally, Mr. Fielding reflects before telling Aziz he will be returning to England:

“ “It is no good,” he thought, as he returned past the mosque, “we all build upon sand; and the more modern the country gets, the worse’ll be the crash. In the old eighteenth century, when cruelty and injustice raged, an invisible power repaired their ravages. Everything echoes now; there’s no stopping the echo. The original sound may be harmless, but the echo is always evil.” This reflection about an echo lay at the verge of Fielding’s mind. He could never develop it. It belonged to the universe that he had missed or rejected. And the mosque missed it too. Like himself, those shallow arcades provided but a limited asylum. “There is no God but God” doesn’t carry us far through the complexities of matter and spirit; it is only a game with words, really, a religious pun, not a religious truth.”