A cool, damp wind, laden with spray, whistled up the tunnel, and the drone grew in volume as the submarine swept on.
A puzzled expression passed over Haverly’s features as he stood listening for some moments.
Then his brow cleared and he slammed to the door.
“I guess we’re nearing the end,” he said; “it’s the sound of a waterfall.”
His comrades gazed despairingly into each other’s faces. What they had feared for so long was about to happen.
Somewhere, not far ahead, the river thundered into space over the brink of some subterranean precipice, and towards this spot the Seal was racing.
The water hissed and foamed about her stern, and long lines of bubbles, gleaming like pearls beneath the searchlight’s glare, danced far ahead, to lose themselves in the darkness of the tunnel.
And ever the drone grew louder, moment by moment, until the Seal, flashing round a curve, swept out into a huge, arched cavern, and the droning note changed to a thunderous roar—the voice of a mighty cataract!
Every plate, every rib which went to form the vessel’s frame, sang with the vibration of the falling waters.
Ahead, the watchers could see the waters leaping, tumbling, foaming in mad confusion, and, beyond, a mighty cloud of mist hovered, veiling, like a white curtain, the terrors of the fearful abyss into which the river plunged.