For a week the _Seal_ remained aground on the mud-bank, while Garth and the engineer, often up to the waist in water, thoroughly overhauled her, fixing duplicate propellers in place of those broken, and replacing the shattered glass with new panes from the store-room. Numerous minor damages which the _Seal_ had sustained in her leap they also repaired. And over them, while they worked, Haverly and the baronet took turns on guard, but no further sign came from the river-creatures, save that once a hideous head rose out of the mud fifty feet from the _Seal,_ to vanish like a flash ere Seymour, who was on guard at the time, could draw trigger. No attack followed this appearance, however, and at length all was completed. The last rivet had been driven into place, the last bolt fixed, and nothing remained but to get the _Seal_ afloat once more. Grasping the wheel, Haverly signalled for full speed astern; the propellers began to revolve, and, slowly but surely, the submarine glided off the mud-bank into deep water. An instant’s pause while the engines were reversed, and then the _Seal_ moved forward on the bosom of the subterranean river at ten knots to the hour. Between the heaving mud-flats she glided, from the surfaces of which arose a nauseous odour of decaying matter, and a dense, malarial vapour ascended, to lose itself in the inky darkness that veiled the cavern roof. For here neither walls nor roof were visible. Nought met the eye but the water--wherein slimy water-snakes writhed and twisted--and the seething mud. Scarce a wave rippled the placid surface of the stream, save those occasioned by the passage of the _Seal,_ and not a sound broke the profound stillness of the vast cavern but the purring note of the engines. So two days went by, with nothing to disturb the dreary monotony of the depressing voyage. Ever the same muddy, grey prospect stretched before the explorers, and they had begun to wonder whether they should ever find a way out of this loathsome river, when something happened. Haverly was at the wheel, the others being below, engaged in their several duties, when a shout brought them rushing into the turret. “Look!” cried the American, pointing ahead. The _Seal_ had passed out of the river, and, before them, shimmering in the rays of the searchlight, rolled a vast, subterranean sea. To starboard, a cable length away, a low, sandy shore was visible, clothed almost to the water’s edge with a weird and curious vegetation which sparkled and gleamed with a dazzling lustre. Flinging open the door, Seymour stepped out on deck, quickly followed by Garth and the professor. “The heart of the globe!” the latter cried excitedly. “A subterranean world! My friends, we have the honour to be the discoverers of an unknown world. Steer her close in, Silas; I am curious to know what manner of growths those are.”