The roof was only a foot above their heads.

The width of the place averaged six or seven feet, and there was this to encourage them--no branches occurred to form puzzling labyrinths.

If they had been overtaken by darkness there was nothing to prevent their feeling their way back into the sunshine.

So, growing accustomed to the place, familiarity, if it did not breed contempt, made them cooler and more ready to go on descending over similar obstacles to those they had previously encountered, till all at once Mike stopped short, and held up the lanthorn beneath which he peered.
"What is it ?" said Vince anxiously.
"Hark! What's that ?" said Mike, in a whisper full of awe.
A dull rushing sound smote upon their ears, but in a muffled, strange way, that puzzled them to make out what it might be.
"I know," said Vince at last: "it's water." "Think so ?" said Mike dubiously.
"Yes.

I've been puzzling ever so long to make out how it was that water could have run along here, and for there to be none now, but I see how it is.