“That comes from the volcano,” he said.

Besides these sounds, the presence of chemical combinations was soon betrayed by their powerful odor, and the engineer and his companion were almost suffocated by sulphurous vapors.

“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 vapor, 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!”

Ayrton said not a word, but, upon a sign from Cyrus Harding, resumed the oars, and half an hour later the engineer and he reached the entrance of Dakkar Grotto.

Chapter 19

The next day, the 8th day of January, after a day and night passed at the corral, where they left all in order, Cyrus Harding and Ayrton arrived at Granite House.

The engineer immediately called his companions together, and informed them of the imminent danger which threatened Lincoln Island, and from which no human power could deliver them.

“My friends,” he said, and his voice betrayed the depth of his emotion, “our island is not among those which will endure while this earth endures.  It is doomed to more or less speedy destruction, the cause of which it bears within itself, and from which nothing can save it.”

The colonists looked at each other, then at the engineer.  They did not clearly comprehend him.

“Explain yourself, Cyrus!” said Gideon Spilett.

“I will do so,” replied Cyrus Harding, “or rather I will simply afford you the explanation which, during our few minutes of private conversation, was given me by Captain Nemo.”

“Captain Nemo!” exclaimed the colonists.

“Yes, and it was the last service he desired to render us before his death!”

“The last service!” exclaimed Pencroft, “the last service!  You will see that though he is dead he will render us others yet!”