"You can see for yourself.

All you've got to do is to find a path leading down, and to follow it.
There's a path over there just in front of you; you can see the opening from where you are lying." He looked, and sure enough there _was_ an opening, and a dim passage running down through the solid rock.

Up he jumped, fired at the prospect of seeing new and wonderful things, and without looking any more to see who had spoken to him, he ran over to it.

The passage had a smooth floor of stone, and sloped downward into the earth, and went round and round in an immense spiral; but the circles were so wide that Martin scarcely knew that he was not travelling in a straight line.

Have you by chance ever seen a buzzard, or stork, or vulture, or some other great bird, soaring upwards into the sky in wide circles, each circle taking it higher above the earth, until it looked like a mere black speck in the vast blue heavens, and at length disappeared altogether?
Just in that way, going round and round in just such wide circles, lightly running all the time, with never a pause to rest, and without feeling in the least tired, Martin went on, only down and down and further down, instead of up and up like the soaring bird, until he was as far under the mountain as ever any buzzard or crane or eagle soared above it.
Thus running he came at last out of the passage to an open room or space so wide that, look which way he would, he could see no end to it.