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.