As we advanced up the river which winds beneath the Golden Cliffs out of the bowels of theMountains of Otz to mingle its dark waters with the grim and mysterious Iss the faint glowwhich had appeared before us grew gradually into an all-enveloping radiance.The river widened until it presented the aspect of a large lake whose vaulted dome, lightedby glowing phosphorescent rock, was splashed with the vivid rays of the diamond, the sapphire, the ruby, and the countless, nameless jewels of Barsoom which lay incrusted in the virgin goldwhich forms the major portion of these magnificent cliffs.Beyond the lighted chamber of the lake was darkness-what lay behind the darkness I couldnot even guess.To have followed the thern boat across the gleaming water would have been to invite instantdetection, and so, though I was loath to permit Thurid to pass even for an instant beyond mysight, I was forced to wait in the shadows until the other boat had passed from my sight at the farextremity of the lake.Then I paddled out upon the brilliant surface in the direction they had taken.When, after what seemed an eternity, I reached the shadows at the upper end of the lake Ifound that the river issued from a low aperture, to pass beneath which it was necessary that Icompel Woola to lie flat in the boat, and I, myself, must need bend double before the low roofcleared my head.Immediately the roof rose again upon the other side, but no longer was the way brilliantlylighted. Instead only a feeble glow emanated from small and scattered patches of phosphorescentrock in wall and roof.Directly before me the river ran into this smaller chamber through three separate archedopenings.Thurid and the therns were nowhere to be seen-into which of the dark holes had theydisappeared? There was no means by which I might know, and so I chose the center opening asbeing as likely to lead me in the right direction as another.Here the way was through utter darkness. The stream was narrow-so narrow that in theblackness I was constantly bumping first one rock wall and then another as the river woundhither and thither along its flinty bed.Far ahead I presently heard a deep and sullen roar which increased in volume as I advanced, and then broke upon my ears with all the intensity of its mad fury as I swung round a sharp curveinto a dimly lighted stretch of wate