Blinded, flung against his metal strap by the resistance of the waters they ripped through, Norman yet retained enough of consciousness to glimpse beams of light that stabbed ahead from the prows of their rushing boats, to see vaguely strange creatures of the deep blundering in and out of those beams as the boats hurtled forward. The water that forced its way between his lips was fresh, he was vaguely aware, and even as he fought to hold his breath was aware too that the frog-men seemed in no way incommoded by the sudden transition into the water, their amphibian nature allowing them to stay under it far longer than any human could do. The boats ripped through the waters at terrific speed and in a few seconds there loomed before them the giant metal wall of the great dome, going down into the depths here. Norman glimpsed vaguely that the whole colossal dome rested on a vast pedestal-like mountain of rock that rose from the sea's floor almost to the surface. Then a great round opening in the wall; the boats flashed into it and were hurtling along a water-filled tunnel. Norman felt his lungs near bursting--when the tunnel turned sharply upward and the boats whizzed up and abruptly out of the water-tunnel into air! * * * * * But it was not the open air again. They were beneath the gigantic dome! For as Norman and Hackett breathed deep, awe fell on their faces as they took in the scene. Far overhead stretched the dome's colossally curving roof, and far out on all sides. It was lit beneath that roof by a clear light that the two would have sworn was sunlight. The dome was in effect the roof of a gigantic, illuminated building, and upon its floor there stretched a mighty city. The city of the frog-men! Their boats were rising up over it and Norman and Hackett saw it clear. Square mile upon square mile of structures stretched beneath the dome, black buildings often of immense size, varying in shape, but all of square, rectangular proportions. Between them moved countless frog-hordes, swirling throngs in streets and squares, and over the roofs darted thick swarms of flying-boats. And at the city's center, in a great, circular, clear space, lay a wide, round, green pool--the opening of the water-tunnel up through which they had come. Norman pointed down toward it. "That's your answer!" he cried. "The only entrance to this frog-city is from the sea, up through that water-tunnel!" "Good God, an amphibian city!" Hackett was shaken, white-faced. The two boats were driving quickly over the city, through the swarming craft. Norman glimpsed towering buildings that might have been palaces, temples, laboratories. They slowed and dipped toward one block-like building not far from the water-tunnel's opening. Armed frog-guards were on its roof, and other boats rested there. The two came to rest and the two captives were jerked out, the guards seizing them.