But when Tuor had lived thus in solitude as an outlaw for four years, Ulmo set it in his heart to depart from the land of his fathers, for he had chosen Tuor as the instrument of his designs; and leaving once more the caves of Androth he went westwards across Dor-lómin. But even as dim dusk came on the third day of his journey, Tuor found before him a wall of rock, and there was an opening therein like a great arch; and the stream passed in and was lost. Then Tuor was dismayed, and he said: "So my hope has cheated me! The sign in the hills has led me only to dark end in the midst of the land of my enemies." But even as the light of the coming sun shone pale in the far mists of Mithrim, Tuor heard voices, and looking down he saw in amazement two Elves that waded in the shallow water. So he found Annon-in-Gelydh, the Gate of the Noldor, which the people of Turgon built when they dwelt in Nevrast long years before.
Thence a dark tunnel led beneath the mountains, and issued into Cirith Ninniach, the Rainbow Cleft, through which a turbulent water ran towards the western sea. The sun rose behind his back and set before his face, and where the water foamed among the boulders or rushed over sudden falls, at morning and evening rainbows were woven across the stream. High hills now marched on either side, and fresh waters spilled from them into Cirith Ninniach over shimmering falls. Then he lifted up his voice, and plucked the strings of his harp, and above the noise of the water the sound of his song and the sweet thrilling of the harp were echoed in the stone and multiplied.
And Tuor came into Nevrast, and looking upon Belegaer the Great Sea he was enamoured of it, and the sound of it and the longing for it were ever in his heart and ear, and an unquiet was on him that took him at last into the depths of the realms of Ulmo. Then he dwelt in Nevrast alone, and the summer of that year passed, and the doom of Nargothrond drew near; but when the autumn came he saw seven great swans flying south, and he knew them for a sign that he had tarried overlong, and he followed their flight along the shores of the sea. The Silmarillion (chapter 23), The Unfinished Tales (Of Tuor and his Coming to Gondolin)