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. “Hold tight!” roared Haverly, his voice ringing clear and true above the din of the falling waters. The others gazed, half fascinated, in spite of the peril at the scene before them. Swiftly the vessel sped on to her doom, the dancing waves lapping her hull playfully as they hurried her forward. Helpless as a log, the splendid craft was turned and twisted in the grip of the cataract. She paused for an instant as she reached the verge, like some terrified animal shying from a leap; then a tremor passed through her plates, and she plunged swiftly over into the depths. Pale as death were her crew, yet never a cry escaped them as their stout vessel pitched downward, stern in air. Through each man’s mind ran the same question: was there deep water beneath the fall, or a row of jagged rocks, on whose giant teeth the unfortunate _Seal_ would shatter herself into a thousand fragments! The time seemed interminable! Would she never stop falling? In reality a few seconds only were occupied by the descent, but to the explorers ages seemed to pass, ere, with a terrible crash, the submarine struck the foaming whirlpool below the cataract. High above the boom of the waters sounded the shock of that fall, and a huge column of spray was flung upward by the impact of the vessel’s hull. Her crew, shaken from their hold, were hurled like puppets against the walls of the turret, and a merciful oblivion once more swept over them. Quickly the vessel was beaten downwards by the enormous weight of the plunging water. Lower and still lower she went, whirling madly, until it seemed as though she would never rise again. Thrice she was swept round in the grip of the whirlpool, only to be drawn back once more to the foot of the fall, as the needle is drawn to the magnet. By some miraculous chance she escaped collision with the rocky walls which formed the basin of the boiling cauldron, although many times within an ace of destruction. Then she was once more swept forward, and this time, escaping the power of the eddy, sped out into the river beyond. A mile lower down she came to the surface and drifted on, her searchlight gleaming through the darkness like the eye of some huge aquatic monster. Hour after hour passed, and still she was borne gently forward on the bosom of the subterranean river. The roar of the fall died to a murmur as she floated on, and at length ceased altogether.