But how long was it, how much more of this would they have to endure, or could they endure? The breathlessness of the air was growing as they climbed; and now they seemed often in the blind dark to sense some resistance thicker than the foul air.

As they thrust forward they felt things brush against their heads, or against their hands, long tentacles, or hanging growths perhaps: they could not tell what they were. And still the stench grew. It grew, until almost it seemed to them that smell was the only clear sense left to them, and that was for their torment. One hour, two hours, three hours: how many had they passed in this lightless hole? Hours - days, weeks rather. Sam left the tunnel-side and shrank towards Frodo, and their hands met and clasped, and so together they still went on.

At length Frodo, groping along the left-hand wall, came suddenly to a void.

Almost he fell sideways into the emptiness. Here was some opening in the rock far wider than any they had yet passed; and out of it came a reek so foul, and a sense of lurking malice so intense, that Frodo reeled. And at that moment Sam too lurched and fell forwards.

Fighting off both the sickness and the fear, Frodo gripped Sam’s hand. ‘Up!’ he said in a hoarse breath without voice. ‘It all comes from here, the stench and the peril. Now for it! Quick!’

Calling up his remaining strength and resolution, he dragged Sam to his feet, and forced his own limbs to move. Sam stumbled beside him. One step, two steps, three steps - at last six steps. Maybe they had passed the dreadful unseen opening, but whether that was so or not, suddenly it was easier to move, as if some hostile will for the moment had released them. They struggled on, still hand in hand…

— J.R.R. Tolkien. Lord of the Rings: The Two Towers