On they went, up the steep acclivity, scrambling over its rugged projections, scaling the irregularities of its gigantic strata, bounding from point to point with the agility of chamois, but never alighting on anything except on the accumulation of the same hexagonal prisms with which they had now become so familiar.
Their exertions were happily rewarded.

Behind a huge pyramidal rock they found a hole in the mountain-side, like the mouth of a great tunnel.
Climbing up to this orifice, which was more than sixty feet above the level of the sea, they ascertained that it opened into a long dark gallery.

They entered and groped their way cautiously along the sides.
A continuous rumbling, that increased as they advanced, made them aware that they must be approaching the central funnel of the volcano; their only fear was lest some insuperable wall of rock should suddenly bar their further progress.
Servadac was some distance ahead.
"Come on!" he cried cheerily, his voice ringing through the darkness, "come on! Our fire is lighted! no stint of fuel! Nature provides that! Let us make haste and warm ourselves!" Inspired by his confidence, the count and the lieutenant advanced bravely along the unseen and winding path.

The temperature was now at least fifteen degrees above zero, and the walls of the gallery were beginning to feel quite warm to the touch, an indication, not to be overlooked, that the substance of which the rock was composed was metallic in its nature, and capable of conducting heat.
"Follow me!" shouted Servadac again; "we shall soon find a regular stove!" Onwards they made their way, until at last a sharp turn brought them into a sudden flood of light.

The tunnel had opened into a vast cavern, and the gloom was exchanged for an illumination that was perfectly dazzling.