Down they sank, into this. First up to their ankles, then to their knees, then to their waists. "We shall be drowned!" cried Dave. "It looks like it," gasped Bob. "Heaven save us!" Soon the water was up to their necks and still the stuff under them continued to sink. Were they to be drowned like rats in a trap? CHAPTER IX A PRISONER UNDERGROUND "What's to do now?" The question came from Bob Vilett. The sinking of the brushwood had ceased, and he and Dave found themselves in water almost up to their chins, in absolute darkness. "I'm sure I don't know, Bob," was the young diver's response. "We are in a tight box, and no mistake." "We can't stay here forever." "True, but if we make a move we may sink deeper than ever, and then it will be all up with us." A dead silence of several minutes followed. Presently both of the lads grew desperate. "We'll have to do something, that's certain," Dave began, when of a sudden the driftwood sank once more, and they found themselves struggling wildly in the black waters at the bottom of the hole. They were soon over their heads, and now found a strong current carrying them they knew not where. They had hold of each other, but soon the force of the water wrenched them apart. Down and down went Dave, and turned over half a dozen times. He felt as if he must be journeying toward the center of the earth, when he reached out his hand and struck a series of smooth rocks. He tried to hold fast, but this was impossible, and in a twinkle he turned over again, and then his feet struck on something of a sandy beach. Hardly knowing what he was doing, he stepped forward, and then found himself clear of the water. This set him to running, and on he went until he brought up with much force against a stone wall, and fell back partly stunned. His feet lay in the water, but his head was on the sand, and thus he remained for fully a quarter of an hour, unable to move. There was a strange ringing in his ears, and when he at last arose his head ached as if it would split open. "Oh!" he groaned, and staggered up the sand to the smooth, rocky wall. Then he fell again, and did not move until half an hour later, when his head felt somewhat better. Where was he, and how could he save himself? These questions were easy to ask, but no answer was at hand, and he sank down much disheartened. Then he suddenly roused himself and called loudly: "Bob! Bob Vilett! Where are you?" Again and again his voice was raised, but only a dismal echo answered him. Was his late companion dead? It was more than likely. The tears sprang unbidden to the young diver's eyes, but he dashed them away. He must save himself, no matter what the cost. He realized that he had been saved from death by drowning only because he was used to being under water a long time without taking a breath.