dust-covered barrier, half expecting it to turn to dust; but evidently
it had been made of the time-defying alloy. It stood firm. And to all
appearances it was nearly air-tight.

"Well!" said the doctor suddenly, so that the other two started
nervously. "The door's got to come down; that's all!" They looked
around; there was no furniture, no loose piece of material of any kind.
Van Emmon straightway backed away from the door about six feet, and the
others followed his example.

"All together!" grunted the geologist; and the three aluminum-armored
monsters charged the door. It shook under the impact; a shower of dust
fell down; and they saw that they had loosened the thing.

"Once more!" This time a wide crack showed all around the edge of the
door, and the third attempt finished the job. Noiselessly--for there was
no air to carry the sound--but with a heavy jar which all three felt
through their feet, the barrier went flat on the floor beyond.

At the same instant a curious, invisible wave, like a tiny puff of wind,
floated out of the darkness and passed by the three men from the earth.
Each noticed it, but neither mentioned it at the time. Van Emmon was
already searching the darkness with the torch.

Apparently it was only an anteroom. A few feet beyond was another wall,
and in it stood another door, larger and heavier than the first. The
three did not stop; they immediately tried their strength on this one
also.

After a half dozen attempts without so much as shaking the massive