"Worse than a big well.

Let's see how deep it is." He stepped back and picked up a stone that had fallen from the roof, returning to where Mike held up the lanthorn for him to see.
Down went the block of stone, and they prepared themselves to hear it go bounding and echoing far away in the bowels of the earth; but it stopped instantly with a loud clang, and Vince cried,-- "Why, it isn't deep at all! I can see it." A ring or two of the rope was cast loose, passed through the handle of the lanthorn, and upon lowering it down block after block presented itself sufficient to enable them to descend into what proved to be quite a hollow, from which the stream must have leapt into another and again into another, each being a fall of only a few feet.

After which there was another great pot-hole, like a vast mortar with a handleless pestle of rock remaining therein.
Beyond this the water had carved out a rugged trough, steep enough to form a slide if they had felt disposed to trust themselves to it, and Vince laughingly suggested that they should glide down.
"Only it wouldn't do," he added.

"We can't tell what's at the bottom.
Might mean a bad fall.

Had enough of it ?" "Yes, ever since we started," replied Mike.
"Then you want to go back ?" "Oh no, I don't," retorted Mike.