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."

From the forward cockpit came a rustling sound. There was a slight jar
in the ship, and it gave as though a weight had been applied to one
side.

"What are you doing, Garland?" asked the doctor sharply.

There was no reply. Again came the rustling sound. The ship gave a
sudden lurch as though a weight had left the side. Carnes suddenly
spoke.

"Good-by, Doctor," he said. "I'm going over the side."

"I have been