"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 affair--"It's no use," panted the geologist, wishing that he could get a handkerchief to his forehead. "We can't loosen it without tools." Jackson was for trying again, but the doctor agreed with Van Emmon. They reflected that they had been away from Smith long enough, anyhow. The cube was out of sight from where they were. Van Emmon turned the light on the walls of the anteroom, and found, on a shelf at one end, a neat pile of those little reels, eleven in all. He pocketed the lot. There was nothing else. Jackson and Kinney started to go. They retreated as far into the main room as their telephone wires would allow. Still the geologist held back. "Come on," said the doctor uneasily. "It's getting cold." Next second they stopped short, nerves on edge, at a strange exclamation from Van Emmon. They looked around to see him pointing his light directly at the floor. Even in that unnatural suit of mail, his attitude was one of horror. "Look here," he said in a low, strained voice. They went to his side, and instinctively glanced behind them before looking at what lay in the dust. It was the imprint of an enormous human foot. * * * * * The first thing that greeted the ears of the explorers upon taking off their suits in the sky-car, was the exultant voice of Smith. He was too excited to notice anything out of the way in their manner; he was almost dancing in front of his bench, where the unknown machine, now reconstructed, stood belted to a small electric-motor. "It runs!" he was shouting. "You got here just in time!" He began to fumble with a switch.