Near the top, however, it didn't look so simple. He was hiding in a clump of tall weeds, face to face with those guns--and nobody knew what other deadly devices. He was stumped as to how he should try to advance further. Make a rush? There was a pretty good chance of getting past the guns that way, as far as he could tell by visual inspection; but surely there'd be something there, in the narrow gaps between the guns--something to kill him, or at least detect his presence! It made his flesh crawl; but need gave his wits a sharper edge. He had to get through, somehow! He searched the line of fantastic, flame-spewing weapons avidly. A hundred yards away there was a small break in it, where an aerial bomb, dropped by one of the planes, had struck. The crater still smoked with the vapors of the explosive. If there was any detecting device there, any taut-stretched wire, or anything that would bring some death machine into play at his accidental touch, it would be shattered, now, and still unrepaired. Scrambling from bush to bush during intervals of darkness, as before, he got to the break in the line, and through it safely. Thus, he looked at last over the hilltops, and down into the area enclosed by that great, mounded rectangle. It was a queer, contrasting scene. Familiar farm buildings stood out in the weird illumination. But everywhere there were mounds of earth and deep pits. From some of the latter, red-lit smoke trailed up toward the stars. Massive things, not unlike army tanks, moved in circles, as if pacing beats, and there was the muffled clang of what could be buried factories. The old fortress had come to life once more, resurrecting itself from its bed of Carboniferous slumber. It was a camp, bristling with strange armaments and bustling with activity. CHAPTER IV Into the Robot's Lair Perry lay prone in the high grass. He was panting and tired, and he felt a little sick again. He knew that whatever chances he had of accomplishing any good here, would be diminished if he waited. There were dozens of ways of getting uselessly killed. So far he hadn't encountered any of that corrosive gas, but hisses, and distant human screams from the flats along the river, told him that it was being used. And though he had his oxygen mask, his clothing and skin could be eaten away and his blood poisoned. Two bombers burst overhead, their powdered wreckage silvery in paths of searchlights. Perry knew he might even be destroyed by the weapons of his own countrymen. [Illustration: Wilcox slipped stealthily past the great robot gun.] So his gaze settled feverishly on the nearest opening in the ground. It wasn't far away, and its depths were lost in darkness. But twice he saw crawling mechanical things emerge from it. It must lead, then, toward the heart of the mystery he was trying to probe.