He knew where the passage led out to.

What lay ahead he could not tell.
If it was a chamber, as he expected, there might quite possibly be other passages leading out of it.

And so it would be well to make sure of recognizing this one again before he loosed his hold on it.

So he pulled off one boot, and feeling carefully round the opening, placed it just inside as a landmark.
Then he groped on along the right-hand wall to learn the size of the chamber, and was immediately thankful that his own passage was safely marked, for he came on another opening, and another, and another, and labelled them carefully in his mind, "One, two, three." It was truly eerie work, groping there in that dense darkness and utter silence, and trying to the nerves even of one who had never known himself guilty of such things.

But, being there, he was determined to learn all he could.
He clung to his right-hand wall as to a life-rope.