When the first glimmers of day filtered grey amid the mists of Dimbar they crept back into the Dry River, and soon after its course turned eastward, winding up to the very walls of the mountains; and straight before them there loomed a great precipice, rising sheer and sudden from a steep slope upon which grew a tangled thicket of thorn-trees. Into this thicket the stony channel entered, and there it was still dark as night; and they halted, for the thorns grew far down the side of the gully, and their lacing branches were a dense roof above it, so low that often Tuor and Voronwë must crawl under like beasts stealing back to their lair.
But at last, as with great labour they came to the very foot of the cliff, they found an opening, as it were the mouth of a tunnel worn in the hard rock by waters flowing from the heart of the mountains. They entered, and within there was no light, but Voronwë went steadily forward, while Tuor followed with his hand upon his shoulder, bending a little, for the roof was low. Thus for a time they went on blindly, step by step, until presently they felt the ground beneath their feet had become level and free from loose stones. Then they halted and breathed deeply, as they stood listening. The air seemed fresh and wholesome, and they were aware of a great space around and above them; but all was silent, and not even the drip of water could be heard. It seemed to Tuor that Voronwë was troubled and in doubt, and he whispered: 'Where then is the Guarded Gate? Or have we indeed now passed it?'
'Nay,' said Voronwë. 'Yet I wonder, for it is strange that any incomer should creep thus far unchallenged. I fear some stroke in the dark.'
But their whispers aroused the sleeping echoes, and they were enlarged and multiplied, and ran in the roof and the unseen walls, hissing and murmuring as the sound of many stealthy voices.
J.R.R. Tolkien ~ Unfinished Tales: The First Age