With their fine senses intensely alert, they were able to make fair progress, even though unaided by their eyes. But Grom checked his advance abruptly. He had a perception of some obstacle before him. He reached out his spear as far as he could. It touched a soft object. The object, whatever it was, surged violently beneath the touch. His flesh crept, and the shaggy hair uplifted on his neck. "Back!" he hissed, thrusting A-ya off to arm's length and bracing his spear point before him to receive the expected attack. A pair of faintly phosphorescent eyes, small, but so wide apart as to show that their owner's head must have been enormous, flashed round upon them. There was a hoarse squeal of alarm, and a heavy body went floundering off into the water. They could hear it swimming away in hot haste. Every one drew a long breath. Then, after a few moments, A-ya laughed softly: "It's good to find something at last that runs away from us instead of after us!" said she. A little further on the cave wall turned to the left. A few steps, and their path came to an end. There was water ahead of them, and on both sides. Grom's exploring spear assured them that it was deep water. "We must swim," said he. "Leave your clubs behind." And leading the way down into the unknown tide, he struck out straight ahead. It was nerve-testing work swimming thus through that unseen water to an unguessed goal; but Grom was unhesitating, and his companions rested upon his steady will. The water was of a summer warmth, and slightly salt, which convinced him that it had free communication with the sunlit tides outside. Several times he came within touch of the rocky walls of the cavern, and found that they went straight down to a depth he could not guess. But he kept on with hope and confidence at a leisurely pace, which, in that bland and windless flood, he knew that every member of his party could have maintained for half a day. Suddenly there appeared ahead of them a faint, bluish gleam upon the water's surface. It was something elusive and unreal, and vaguely menacing. "Daylight!" exclaimed young Mo eagerly. But Grom said nothing. He did not think it was daylight, and he was apprehensive of some new peril. The strange light grew and spread. It was evident now that it rose from the water, and also that it was advancing rapidly to meet the astonished swimmers. After a few moments it was bright enough in its blue pallor to show the swimmers that they were traversing a vast hall of waters, whose roof was lost in darkness. Some fifty yards ahead of them, and a little to the right, a low spit of rock, half awash for the greater part of its length, ran out slantingly from the wall of the stupendous chamber.