“In an hour the opening will be practicable.”

“It is there, then?” said Pencroft.

“Did you doubt it?” returned Harding.

“But this cavern must be filled with water to a certain height,” observed Herbert.

“Either the cavern will be completely dry,” replied Harding, “and in that case we can traverse it on foot, or it will not be dry, and some means of transport will be put at our disposal.”

An hour passed.  All climbed down through the rain to the level of the sea.  There was now eight feet of the opening above the water.  It was like the arch of a bridge, under which rushed the foaming water.

Leaning forward, the engineer saw a black object floating on the water.  He drew it towards him.  It was a boat, moored to some interior projection of the cave.  This boat was iron-plated.  Two oars lay at the bottom.

“Jump in!” said Harding.

In a moment the settlers were in the boat.  Neb and Ayrton took the oars, Pencroft the rudder.  Cyrus Harding in the bows, with the lantern, lighted the way.

The elliptical roof, under which the boat at first passed, suddenly rose; but the darkness was too deep, and the light of the lantern too slight, for either the extent, length, height, or depth of the cave to be ascertained.  Solemn silence reigned in this basaltic cavern.  Not a sound could penetrate into it, even the thunder peals could not pierce its thick sides.

Such immense caves exist in various parts of the world, natural crypts dating from the geological epoch of the globe.  Some are filled by the sea; others contain entire lakes in their sides.  Such is Fingal’s Cave, in the island of Staffa, one of the Hebrides; such are the caves of Morgat, in the bay of Douarnenez, in Brittany, the caves of Bonifacio, in Corsica, those of Lyse-Fjord, in Norway; such are the immense Mammoth caverns in Kentucky, 500 feet in height, and more than twenty miles in length!  In many parts of the globe, nature has excavated these caverns, and preserved them for the admiration of man.

Did the cavern which the settlers were now exploring extend to the center of the island?  For a quarter of an hour the boat had been advancing, making detours, indicated to Pencroft by the engineer in short sentences, when all at once,—­

“More to the right!” he commanded.

The boat, altering its course, came up alongside the right wall.  The engineer wished to see if the wire still ran along the side.

The wire was there fastened to the rock.

“Forward!” said Harding.

And the two oars, plunging into the dark waters, urged the boat onwards.

On they went for another quarter of an hour, and a distance of half-a-mile must have been cleared from the mouth of the cave, when Harding’s voice was again heard.

“Stop!” said he.

The boat stopped, and the colonists perceived a bright light illuminating the vast cavern, so deeply excavated in the bowels of the island, of which nothing had ever led them to suspect the existence.