Professor Stevens was the first to speak. "This is unbelievable!" he said. "I cannot credit it. We must have lost our senses." "Or our bearings!" added Diane, more hopefully. "Suppose we look around the other side." As for Larry, a darker suspicion flashed through his mind. Captain Petersen! Had he seized his opportunity and led the crew to mutiny, in the hope of converting the expedition into a treasure hunt? Was that the reason he had been so willing to remain behind? He kept his suspicion to himself, however, and accompanied Diane and her father on a complete circuit of the pyramid; but, as he feared, there was no sign of the _Nereid_ anywhere. The craft had vanished as completely as though the ocean floor had opened and swallowed her up. But no, not as completely as that! For presently the professor, who had proceeded to the site where they left the craft resting on the sand, called out excitedly: "Here--come here! There are tracks! Captain Petersen was right! They were attacked!" Hurrying to the scene, they saw before them the plain evidences of a struggle. The ocean bottom was scuffed and stamped, as though by many feet, and a clear trail showed where the craft had finally been dragged away. Obviously there was but one thing to do and they did it. After a brief conference, they turned and followed the trail. * * * * * It led off over the plateau a quarter mile or more, in an eastward direction, terminating at length beside one of the smaller pyramids--and there lay the _Nereid_, apparently unharmed. But her lights were out and there came no answer to their repeated calls, so they judged she must be empty. What had happened to Captain Petersen and his crew? What strange sub-sea enemy had overcome them? What was now their fate? Unanswerable question! But one thing was certain. Larry had misjudged the captain in suspecting him of mutiny. He was sorry for this and resolved he would make amends by doing all in his power to rescue him and his men, if they were still living. Meanwhile his own plight, and that of Diane and her father, was critical. What was to be done? Suddenly, as all three stood there debating that question, Professor Stevens uttered an exclamation and strode toward the pyramid. Following him with their eyes, they saw him pass through an aperture where a huge block of stone had been displaced--and disappear within. The next moment they had joined him, to find themselves in a small flooded chamber at whose far end a narrow gallery sloped upward at a sharp angle. The floor and walls were tiled, they noted, and showed none of the corrosion of the exterior surfaces. Indeed, so immaculate was the room that it might have been occupied but yesterday. As they stood gazing around in wonder, scarcely daring to draw the natural inferences of this phenomena, there came a rasping sound, and, turning toward the entrance, they saw a massive section of masonry descend snugly into place.