"Well, my dear Cyrus," said Gideon Spilett, "here is a very secure retreat, well hid in the depths of the rock, but it is, however, uninhabitable."
"Why uninhabitable?" asked the sailor.
"Because it is too small and too dark."
"Couldn't we enlarge it, hollow it out, make openings to let in light and air?" replied Pencroft, who now thought nothing impossible.
"Let us go on with our exploration," said Cyrus Harding. "Perhaps lower down, nature will have spared us this labor."
"We have only gone a third of the way," observed Herbert.
"Nearly a third," replied Harding, "for we have descended a hundred feet from the opening, and it is not impossible that a hundred feet further down———"
"Where is Top?" asked Neb, interrupting his master.
They searched the cavern, but the dog was not there.
"Mostly 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 détours, 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,
