They searched the cavern, but the dog was not there.

“Most likely he has gone on,” said Pencroft.

“Let us join him,” replied Harding.

The descent was continued.  The engineer carefully observed all the deviations of the passage, and notwithstanding so many detours, he could easily have given an account of its general direction, which went towards the sea.

The settlers had gone some fifty feet farther, when their attention was attracted by distant sounds which came up from the depths.  They stopped and listened.  These sounds, carried through the passage as through an acoustic tube, came clearly to the ear.

“That is Top barking!” cried Herbert.

“Yes,” replied Pencroft, “and our brave dog is barking furiously!”

“We have our iron-tipped spears,” said Cyrus Harding.  “Keep on your guard, and forward!”

“It is becoming more and more interesting,” murmured Gideon Spilett in the sailor’s ear, who nodded.  Harding and his companions rushed to the help of their dog.  Top’s barking became more and more perceptible, and it seemed strangely fierce.  Was he engaged in a struggle with some animal whose retreat he had disturbed?  Without thinking of the danger to which they might be exposed, the explorers were now impelled by an irresistible curiosity, and in a few minutes, sixteen feet lower they rejoined Top.

There the passage ended in a vast and magnificent cavern.

Top was running backwards and forwards, barking furiously.  Pencroft and Neb, waving their torches, threw the light into every crevice; and at the same time, Harding, Gideon Spilett, and Herbert, their spears raised, were ready for any emergency which might arise.  The enormous cavern was empty.  The settlers explored it in every direction.  There was nothing there, not an animal, not a human being; and yet Top continued to bark.  Neither caresses nor threats could make him be silent.

“There must be a place somewhere, by which the waters of the lake reached the sea,” said the engineer.

“Of course,” replied Pencroft, “and we must take care not to tumble into a hole.”

“Go, Top, go!” cried Harding.

The dog, excited by his master’s words, ran towards the extremity of the cavern, and there redoubled his barking.

They followed him, and by the light of the torches, perceived the mouth of a regular well in the granite.  It was by this that the water escaped; and this time it was not an oblique and practicable passage, but a perpendicular well, into which it was impossible to venture.

The torches were held over the opening:  nothing could be seen.  Harding took a lighted branch, and threw it into the abyss.  The blazing resin, whose illuminating power increased still more by the rapidity of its fall, lighted up the interior of the well, but yet nothing appeared.  The flame then went out with a slight hiss, which showed that it had reached the water, that is to say, the level of the sea.