“That comes from the volcano,” he said.
Besides these sounds, the presence of chemical combinations was soon betrayed by their powerful odour, and the engineer and his companion were almost suffocated by sulphureous vapours.
“This is what Captain Nemo feared,” murmured Cyrus Harding, changing countenance. “We must go to the end, notwithstanding.”
“Forward!” replied Ayrton, bending to his oars and directing the boat towards the head of the cavern.
Twenty-five minutes after entering the mouth of the grotto the boat reached the extreme end.
Cyrus Harding then, standing up, cast the light of the lamp upon the walls of the cavern which separated it from the central shaft of the volcano. What was the thickness of this wall? It might be ten feet or a hundred feet—it was impossible to say. But the subterranean sounds were too perceptible to allow of the supposition that it was of any great thickness.
The engineer, after having explored the wall at a certain height horizontally, fastened the lamp to the end of an oar, and again surveyed the basaltic wall at a greater elevation.
There, through scarcely visible clefts and joinings, escaped a pungent vapour, which infected the atmosphere of the cavern. The wall was broken by large cracks, some of which extended to within two or three feet of the water’s edge.
Cyrus Harding thought for a brief space. Then he said in a low voice—
“Yes! the captain was right! The danger lies there, and a terrible danger!”