"There must be a ventilation shaft somewhere around here," he whispered, his mouth not an inch from Carnes' ear, "and we've got to find it. It would never do to try the door; if any of them are still here it is sure to be guarded. You go up the hill for five yards and I'll go down. Quarter back and forth on a two hundred yard front and work carefully. Don't fall in, whatever you do. We'll return to this point every time we pass it and report." The operative nodded and walked a few yards up the hill and made his way slowly forward. He went a hundred yards as nearly as he could judge and then stepped five yards further up the hill and made his way back. As he passed the starting point he approached and Dr. Bird's figure rose up. "Any luck?" he whispered. Dr. Bird shook his head. "Well try further," he said. "I think it is probably beyond us, so suppose you go fifteen yards up and quarter the same as before." * * * * * Carnes nodded and stole silently away. Fifteen yards up the hill he went and then paused. He stood on the crest of the hill and before him was a steep, almost precipitous slope. He made his way along the edge for a few yards and then paused. Faintly he could detect a murmur of voices. Inch by inch he crept forward, going over the ground under foot. He paused and listened intently and decided that the sound must come from the slope beneath him. A glance at his watch told him that he had spent ten minutes on this trip and he made his way back to the meeting place. Dr. Bird was waiting for him, and in a low whisper Carnes reported his discovery. The doctor went back with him and together they renewed the search. The slope of the hill was almost sheer and Carnes looked dubiously over the edge. "I wish we had brought the parachutes," he whispered to the doctor. "We could have taken the ropes off them and you could have lowered me over the edge." Dr. Bird chuckled softly and tugged at his middle. Carnes watched him with astonishment in the dim light, but he understood when Dr. Bird thrust the end of a strong but light silk cord into his hands. He looped it under his arms and the doctor with whispered instructions, lowered him over the cliff. The doctor lowered him for a few feet and then stopped in response to a jerk on the free end. A moment later Carnes signaled to be drawn up and soon stood beside the doctor. "That's the place all right," he whispered. "The whole cliff is covered with creepers and there is a tree growing right close to it. If we can anchor the cord here, I think that we can slide down to a safe hold on the tree."