was a ripple, flickering across the ground from where much of the smashed rock still lay, in all directions. The quivering of each boulder it passed was quite visible, rapid as the passage was. He realized that the wave had started and gone under the balloon before he heard anything, but only later realized — when it was pointed out to him — that the disturbance must have traveled faster than sound. He didn’t bother to ask why even then. The alien watchers on the satellite realized that the ripple must be a seismic wave, but none had a really good look at it just at first; it was out of the viewer field, close to the horizon as much of that was, in much less than a second. High-speed cameras had recorded it all, of course, but time was needed to play these records back, and there had not yet been time. One of the Mesklinites, favored by a far wider field of vision, called attention to the real results of the blast. “LOOK!” The word was a bellow in Stennish, which Jeanette didn’t bother to translate to her fellows. “Show us! Let us look too!” she cried. “Where? What? Turn the lens!” Sherrer was a little slow in responding, his attention being focused toward a spot at the edge of the cliff, half a mile from the balloon and from where the rock had been, and at right angles to the line of sight between these. Barlennan reached for the rotation valve lines, but the balloon was as usual slow in responding. Starting at the point nearest the blast, the edge of the precipice was starting to crumble away and, of course, to disappear. Cracks nearly parallel to the cliff face and up to a meter from it were appearing. Others nearly perpendicular to them were also showing briefly; then the outer sides of the prisms they outlined were leaning slightly farther out and promptly vanishing. New cracks closer to the balloon than the new edge appeared immediately, outlining sections which were vanishing in turn. The disturbance was spreading in both directions from the point where it had started — the point where, Barlennan realized, the “ripple” must first have reached the cliff face. His memory flashed to the rockfall, hundreds of miles away, where he and some of his crew had first climbed to the plateau tens of thousands of days before. That had not stretched very far along the cliff. Right now his people and vessel, the original Bree, were ten miles along the edge from this new point of collapse and should be safe— If this one spread no farther than the other. Neither he nor any of the Flyers had been able to explain what had caused that other fall. A vertical cliff three hundred feet high and of any length at all near a Mesklin pole was unbelievable enough; the now well determined fact that it completely rimmed a continentsized area of the giant planet was worse. The general layered appearance, which the Flyers claimed to mean sedimentary rock, was hard even for them to reconcile with an unbroken vertical cliff. There should be rock fragments — more reasonably rock powder — ar the bottom. All along the bottom. Barlennan had often heard them arguing about whether the perfectly vertical joints in the cliff face implied that the plateau had been lifted or the surroundings lowered, but it had been another of the inconclusive debates which seldom held his attention for long. No guess at what might have caused that local fall had ever come close to explaining why it had stayed local; it was easy to imagine something like a careless footfall’s (whose?) starting the break — but what could possibly have kept it from dominoing both ways the whole ten- or twelve-thousand-mile circumference of the continent? And for that matter, what had studded so many thousands of square miles of the plateau’s outer edge with boulders up to truck size, most of them lying on the surface rather than even partially buried? No one on Toorey was in the least surprised that Mesklin showed tectonic activity, and no one was too surprised that this differed in detail from anything familiar to human, Drommian, and the other researchers’ experience. The edges of the plateau which had been seen, only a small fraction of its total circumference, did appear to be sedimentary rock, but this did nothing to make theorizing simpler. Would the same unknown cause, or any other, operate to stop the spread of this fall? Were his crew members safe? The original Bree was ashore on the far side of the river, a little farther from the base of the cliff than the scarp itself was high. Many of those below would be away from the ship, farther still from the cliff, hunting, and presumably safe. Others, though, might well be fishing, since the river was a major source of food. There was nothing Barlennan could do. The Flyers were still calling for attention. There was nothing they could possibly do either, but they deserved to watch. Barlennan was a responsible and reasonably fair-minded adult, and never thought of blaming them for what was happening. The slow swiveling of the balloon finally brought the lens to face the cliff edge, not at the nearest point but well to the right, where the unaffected edge itself could still be seen. The captain stopped the rotation there. The new edge was now much closer to the balloon and— And its growth was slowing? Surely it was slowing? Barlennan’s people, after many thousands of days piling dirt and rocks around the alien rocket in the course of business, had a very clear concept of angle of repose. The collapse couldn’t possibly extend much farther back from the original edge than it was now getting, the captain told himself. The whole plateau would never crumble to fragments, obviously. At least, it hadn’t the other time. But that was not the immediate problem. How far along would the disintegration extend? How safe were his other men? And how would he get his people and the stuff the Flyers wanted to recover physically, such as the inertial tracker, back to the equator if his original ship were lost? Taking the balloon across thousands of miles of ocean was ridiculous; it could carry fuel to heat its air for only a few days, in spite of Karondrasee’s endless research into different juices with maybe more effective enzymes. It could not carry the whole crew, even if the stuff wanted by the Flyers were all left behind. “We’ve warned Dondragmer.” Jeanette’s voice caught the captains’s attention at this point in his thoughts. “No one is on the cliff side of the river. A dozen are away hunting. The ones still at the ship are getting as far from the river and the cliff as quickly as they can carry the radio.”