"One's mind," Spurlin interrupted softly, "somehow, does not seem to be the same after three years in this place." "All right. But before _I_ leave here I'm going to find out what Bhruulo's purpose is! I don't like the way that old Martian grinned at me. He's got something up his sleeve, and I think you men'll find it out too late." Spurlin smiled sadly. "All right, Jim Landor. Each man is his own boss here. At least I wish you would accompany a few of us tomorrow. We're getting more of the metal out, and trying to determine the proper spot to blast through with our spacer. You'll become more acquainted with the city and the general terrain, and maybe it'll change your mind." "Sure, I'll go," Jim agreed. But he didn't think it would change his mind. He had wanted to find M'Tonak, here he was in M'Tonak and he was gong to solve the mystery of M'Tonak. More than that, he was going to learn once and for all what had happened to his brother. V The following day a dozen men ventured up into the city. Spurlin seemed disappointed as they stepped out into the street from their secret building. "Not an ideal day for it," he commented gruffly. And at Jim's querulous look, he explained, "Those emanations seem stronger today. I give us only two or three hours, at the most." They went into the rocky terrain beyond the city, toward the near horizon where the cave roof tapered down. That was hardly a mile away. Jim found it hard to believe that over their heads was the Polar Cap, vast and desolate. Glancing up, he barely made out the dim contour of their roof; and it suddenly occurred to him to wonder what sustained it, why it didn't collapse under that tremendous pressure of rock and ice! He knew why, only a minute later. There came a sudden, smooth hum in his ears. The very air around them seemed surcharged with energy, or rather all energy seemed to be rushing _away_ from them! "This way!" Spurlin exclaimed, making a hasty detour from the spot. Barely a hundred yards away Jim could discern a vague swirling mistiness, in the form of a huge column that reached up to touch the roof. Suddenly, he knew what it was, knew also that it would be death for any man who ventured too close. "Ionization zone." Spurlin voiced Jim's own thoughts as they hurried in the detour. "An electronic tower of strength! There are usually six of them in a straight line across this cave, but once in a while new ones spring up out of nowhere. I think Bhruulo controls them." Jim nodded uncomfortably, and tried not to think what would happen if all those electronic zones failed, with millions of tons of ice above them. They reached their objective at last. Tunnels were in evidence where the men had been taking out the ore. They resumed work at once, but it was slow and heart-breaking. Their tools were crude, and the ore was the most difficult Jim had ever handled.