d it is not impossible that a hundred feet farther
down--"

"Where is Top?" asked Neb, interrupting his master.

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 wa