"All right," Spurlin murmured softly. "No harm if he comes. But I don't think he'll last long, no Martian ever does in this city." If Kaarji heard the words he did not show it, as he followed Jim into the tunnel. "About your brother," Spurlin spoke brusquely out of the darkness as they moved along. "Yes, he arrived here all right. For a while, Frank Landor was with our secret little group down here below. But--there's something about that greenish atmosphere, something exhilerating but also deadly, in a very subtle and insidious way. Sometimes it increases, penetrates even down to us, through walls and things. But there are some men who--" "Yes, I know," Jim's voice was as dead as the hope within him. "Frank was one of those men. He couldn't stay cooped up here. He was curious, he had to find out--things, and the reason for things. That what you're trying to tell me?" "That's about it. Like others who have come here he had to go up into the city, searching, trying to solve its secret. Every day he and a few others went up. Always they returned to us here, exhausted, until one day--they just didn't come back." In silence they continued along the winding passage. Jim was thinking of his brother now, with a dawning realization that he would probably never again see him alive. He was thinking of other things too. Of that menacing greenness in the city above. Of Spurlin who seemed so calloused and unconcerned. Of the legendary emerald of M'Tonak, the lure for countless men in ages past. * * * * * Spurlin's voice shattered the silence. "Here we are." Now he was flashing a tiny light upon a massive metal door. And Jim's heart leaped, for he saw it as a metal new, and yet not new to him. It was the same dull, greenish-gray metal as the piece Kaarji had given him. Jim passed his fingers lightly across it to make sure, but said nothing. For more than any of these things he was thinking of a bent and shrivelled old Martian named Bhruulo, who had chuckled with a secret evil glee. The door swung ponderously open. They stepped into a huge oval room, and many men came hurrying toward them. The walls of this room, Jim noticed, were of the same peculiar metal. "Introductions later," Spurlin said, as the men came crowding around. "Right now I want you newcomers to see the work we're engaged in here. You look like the sort who can help us in the job." He led them to another room where a long, skeletal shape was under construction. It rested on curved cradles, pointing upward. Only a few outer plates had as yet been put into place, plates of the same strange metal Jim identified with everything here. "A spaceship!" he exclaimed unbelievingly. "But--why a spacer here, so far beneath Mars' surface?"