He had, somehow or other, brought up in the rear. Though he did not admit it, this was because he had shoved his cousins ahead of him, hoping thus to enable them to gain a safe place. And as Nort and Dick glanced back they saw, in the gleam of the one lantern left alight, a white mass of water bearing down on them, and, seemingly, filling the tunnel from wall to wall, as it rushed foaming and murmuring onward. It was as though a dam had suddenly burst, or some obstruction had been removed, allowing the pent-up waters to rush along the accustomed channel. And if you have ever noticed a dammed-up stream, say in some gutter, thus quickly released, you can imagine what happened on a larger scale in the tunnel where the boys were. The water, normally, flowed only in the four-foot channel. But now it spread out on either side, and, of course, was much deeper in the centre. But as the tunnel sloped from either wall, in a sort of V shape to the centre channel, naturally the parts nearest the side walls were less covered by water than the others. It was because of this that Bud, Nort and Dick were enabled to maintain a footing, though they were knee-deep in water in an instant, and the one remaining lantern had to be held up to prevent it from being engulfed and extinguished in the sudden flood. "Climb up! Climb up!" shouted Bud. "Isn't there some place--some rocky ledge--where you can find a footing? The water's getting deeper!" And this was true. Either the flood was growing at its source (a place as yet unknown to the boys) or it was running too rapidly, and in too great a volume, to accommodate itself to the tunnel channel, and was thus piling up in the vicinity of the boys. "What happened? What caused it?" cried Nort. "Never mind that--now!" shouted Bud. "Find the highest place you can, and stick!" "Suppose the whole tunnel fills?" asked Dick, trying to pierce the semi-gloom, and look for a refuge on the rocky wall. "If it does we'll have to swim for it," grimly said Bud. "But isn't there some place where you can climb up?" "This looks like a ledge," Dick answered, as he caught sight of a darker shadow on the rocky wall of the tunnel, above his head, when his brother swung the lantern. "Just what we need!" exclaimed Bud, as he waded through the ever-deepening water to the side of his cousins. "Up with you! Here, Nort, I'll hold the lantern until you make it!" Thus, again, Bud was seeing that his cousins reached a place of comparative safety before he looked to himself. For they found the ledge, once they had scrambled up to it, well above the water, and wide enough to give shelter and a safe perch for all three. "Whew! That was touch and go!" murmured Bud, as he leaned back, half exhausted, against the rocky wall at the rear of the ledge. "I should say so!" gasped Dick. "It all happened so suddenly that I don't know yet what it was all about."