"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.
hey 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."