CLOSE TO CRITICAL

climbed this quickly, and paused at a narrow gully opening near the top. It seemed to be looking around for followers, though no organs of vision had yet been identified by the human watchers. Apparently satisfied, it drew an ellipsoidal object from its sack, examined it carefully with delicate fingers, and then disappeared into the gully.

In two or three minutes it was back, this time without its grapefruit-size burden. Heading down the hill once more, it avoided with care both its own traps and the others, and set off in a direction different from that of its approach.

The robot's operators had to think fast. Should they follow the native or find out what it had been doing up the hill? The former might seem more logical, since the native was leaving, and the hill presumably was not, but the second alternative was the one they chose. After all, it was impossible for the thing to travel without leaving some sort of trail; besides, night was approaching, so it wouldn't get far. It seemed safe to assume that it shared the characteristic of Tenebra's other animal life, of collapsing into helplessness a few hours after nightfall.

Besides, looking at the hilltop shouldn't take too long. The robot waited until the native was well out of sight, and then moved up the hill toward the gully. This, it turned out, led into a shallow crater, though the hill bore no resemblance to a volcano; on the crater floor lay perhaps a hundred ellipsoids similar to that which the native had just left there. They were arranged with great care in a single line, and except for that fact were the closest things to loose stones that the men had yet seen on Tene-bra. Their actual nature seemed so obvious that no effort was made to dissect one.

At this point there must have been a lengthy and lively discussion. The robot did nothing for quite a long time. Then it left the crater and went down the hill, picked its way carefully out through the "mine field"

on the trail of the native, and settled down to travel.

Prologue: Investigation; Annexation 15

This was not quite as easy as it would have been in the day time, since it was starting to rain and visibility was frequently obstructed by the drops. The men had not yet really decided whether it was better, hi traveling at night, to follow valleys and remain submerged or stick to ridges and hilltops so as to see occasionally; but hi this case the problem was irrelevant. The native had apparently ignored the question, and settled for something as close to a straight line as it could manage. The trail ran for some ten miles, and ended at a clearing before a cave-studded cliff.

Details could not be seen well. Not only was the rain still falling, but the darkness was virtually absolute even to the pickups of the robot. More discussion must have resulted from this; it was two or three minutes after the machine's arrival at the clearing that its lights went on and played briefly over the rock.

Natives could be seen standing inside the cave mouths, but they made no response to the light. They were either asleep, in more or less human fashion, or had succumbed to the usual night-torpor of Tenebra's animal life.

No sign of anything above a stone-age culture level could be seen anywhere about, and after a few minutes of examination the robot cut off most of its lights and headed back toward the hill and the crater.

It moved steadily and purposefully. Once at the hilltop, several openings appeared hi its sides, and from some of these armlike structures were extended. Ten of the ellipsoids were picked carefully from one end of the line— leaving no betraying gaps—and stowed in the robot's hull. Then the machine went back down the hill and began a deliberate search for booby traps. From these it removed the stone blades, and such of these as seemed in good condition—many were badly corroded, and some even crumbled when handled—disappeared into other openings in the lump of plastic. Each of these holes was then covered by a lid of the same incredibly stable

16