Roland did not believe that the earth had a crust.

He had no faith in the old-fashioned idea that the great central portion was a mass of molten matter, but he could not drive from his mind the conviction that his light had passed through the solid portion of the earth, and had emerged into something which was not solid, which was not liquid, which was in fact nothing.
All his labors had come to this: he had discovered that the various strata near the earth's surface rested upon a vast bed of rock, and that this bed of rock rested upon nothing.

Of course it was not impossible that the arrangement of the substances which make up this globe was peculiar at this point, and that there was a great cavern fourteen miles below him; but why should such a cavern be filled with a light different from that which would be shown by his Artesian ray when shining upon any other substances, open air or solid matter?
He could go no deeper down--at least at present.

If he could make an instrument of increased power, it would require many months to do it.
"But I will do it," said he to Margaret.

"If this is a cavern, and if it has a bottom, I will reach it.