Carnes watched the ground below them carefully. There was nothing about it to distinguish it from any other wooded mountainous country and his interest waned. He glanced aloft. The mail plane had disappeared in the distance and the sky was clear of aircraft. He turned again to the ground. It looked closer than it had before. He turned and looked at the duplicate altimeter. The plane had lost nearly a hundred feet elevation. * * * * * "There's something wrong about this plane, Doctor," came Captain Garland's voice through the speaking tube. "It doesn't behave like it should." "I guess we've found what we were looking for, Carnes," said Dr. Bird grimly. "What seems to be the matter, Captain?" "Blessed if I know," was the answer. "It feels like a drag of some sort, like an automobile going through heavy sand. We're slowing down, though I am giving her all the gun I've got!" "Cut your motor!" said the doctor shortly. He bent over the duplicate instrument board as the roar of the motor died away. Carnes rose and looked over the side. "Look, Doctor!" he cried in a strained voice. Directly below them yawned a hole sixty feet in diameter and extending down into the bowels of the earth. The plane hovered over the hole for a moment and then slowly descended into it. "What is it?" cried the detective. "It's the secret of the Port of Missing Planes," replied Dr. Bird. "Throw off your parachute. Keep your gun and light handy but don't fire unless I do first. The same holds good for you, Captain." The plane sunk until it was fifty feet below the level of the ground. Carnes looked up. Gradually the circle of sky became blurred and hazy as though the air were heavy with dust. The rasp of Dr. Bird's flashlight key aroused him and he hastily wound his own. The haze above them grew thicker. Suddenly the light died and then came darkness, a darkness so thick and absolute that it bore down on them like a weight. Dr. Bird's light stabbed a path through it. * * * * * They were in a tunnel or tube reaching into the ground. The sides were smooth and polished, as though water worn. The plane sank deeper and deeper into the earth. Suddenly Dr. Bird's light went out. "What's the matter, Doctor?" asked Carnes, "did your light fail?" "No," came a strained voice. "I turned it out." "Why?" "I don't know. Light yours." Carnes reached into his pocket. Dr. Bird could hear his breath come in panting sobs as though he were exerting his whole strength. "I can't do it, Doctor," he gasped. "I want to, but some power greater than my will prevents me." "Are you affected, Captain?" asked the Doctor. "I--can't--move," came in muffled accents from the front cockpit. "Some power beyond my knowledge has us in its grasp," said the doctor. "All we can do is sit tight and see what happens. We are no longer falling at any rate."