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.