Meanwhile Top had returned at his master’s call, and had regained the shore:  but he could not stay quiet; he plunged in among the tall grass, and guided by instinct, he appeared to follow some invisible being which was slipping along under the surface of the water.  However the water was calm; not a ripple disturbed its surface.  Several times the settlers stopped on the bank, and observed it attentively.  Nothing appeared.  There was some mystery there.

The engineer was puzzled.

“Let us pursue this exploration to the end,” said he.

Half an hour after they had all arrived at the southeast angle of the lake, on Prospect Heights.  At this point the examination of the banks of the lake was considered finished, and yet the engineer had not been able to discover how and where the waters were discharged.  “There is no doubt this overflow exists,” he repeated, and since it is not visible it must go through the granite cliff at the west!”

“But what importance do you attach to knowing that, my dear Cyrus?” asked Gideon Spilett.

“Considerable importance,” replied the engineer; “for if it flows through the cliff there is probably some cavity, which it would be easy to render habitable after turning away the water.”

“But is it not possible, captain, that the water flows away at the bottom of the lake,” said Herbert, “and that it reaches the sea by some subterranean passage?”

“That might be,” replied the engineer, “and should it be so we shall be obliged to build our house ourselves, since nature has not done it for us.”

The colonists were about to begin to traverse the plateau to return to the Chimneys, when Top gave new signs of agitation.  He barked with fury, and before his master could restrain him, he had plunged a second time into the lake.

All ran towards the bank.  The dog was already more than twenty feet off, and Cyrus was calling him back, when an enormous head emerged from the water, which did not appear to be deep in that place.

Herbert recognized directly the species of amphibian to which the tapering head, with large eyes, and adorned with long silky mustaches, belonged.

“A lamantin!” he cried.

It was not a lamantin, but one of that species of the order of cetaceans, which bear the name of the “dugong,” for its nostrils were open at the upper part of its snout.  The enormous animal rushed on the dog, who tried to escape by returning towards the shore.  His master could do nothing to save him, and before Gideon Spilett or Herbert thought of bending their bows, Top, seized by the dugong, had disappeared beneath the water.

Neb, his iron-tipped spear in his hand, wished to go to Top’s help, and attack the dangerous animal in its own element.

“No, Neb,” said the engineer, restraining his courageous servant.