"Everything on this planet does--that is, everything native to it. And they're impervious to heat, of course. If Vulc had not been captured by George Marnik almost immediately after it was born, it would never have been conditioned to the comparatively cool atmosphere of the Base." In silence they trudged mile after mile, following the same line of black hills that housed their Base. Mark marvelled at how comfortable the vacuum suits were, but he knew the real heat hadn't started yet. It came presently, as they veered further outward from the hills. The heat increased steadily and became more intense than anything Mark had ever experienced. Perspiration dripped stickily within his suit. He wanted to wipe his face but couldn't; he could only shake his head to keep the sweat from his eyes. But there was no keeping the mirages from his eyes. In every direction the terrain rocked and rolled under huge undulating hazes of heat. Horizons leaped at him in wave after wave, and fled away again. The men ahead seemed to do fantastic dances. They no longer trod on rock. The ground beneath was soft, white and leprous looking, powdery almost as dust. Mark felt it hot around his metal-clad ankles. Now he realized the reason for the flat-bottomed sleds. He knew, too, that a spaceship could never venture over here and get back safely; compasses and magniplates and everything else would go haywire. Peering ahead, he discerned Vulc's fantastic bulk which now had turned a glowing cherry red! He shuddered at the thought of what would happen to a man suddenly bereft of the protecting vacuum suit. Out of the silence, a vast rumbling sound rose like magnified thunder. Mark saw Carston fumble with his radio-phone then peer all about into the haze. "Blitzees coming!" he yelled into his instrument. Everyone stopped. Mark followed Carston's line of sight, but he couldn't see a thing. "Swarm coming from the left!" Carston yelled again. The Commander moved hurriedly along the line. "Lie down everyone, face to the left! Upend your sleds and if you value your lives, stay behind them!" For a second all was confusion as the men flung themselves to the powdery soil; then a metal barrier sprang up as the sleds came end to end. Still nothing could be seen. * * * * * Suddenly then they came. The air was blue from crackling sparks as dozens of the Blitzees struck the sleds with the impact of bullets. A sound like the humming of millions of hornets was in their ears, as the greater part of the swarm passed overhead. Mark had a confused vision of electric blue streaks that writhed and zig-zagged, landed and leaped again, propelling themselves blindly. As suddenly as it had come, the danger was over.