He started running back toward the Umbilical, while Rebka hesitated. The line of eruptions was curiously orderly, their spreading darkness bursting precisely from every third peak. He gave one quick look the other way — would water be a safer haven? — and then followed Perry. The ground began to shake, to seesaw back and forth so that he was close to losing his balance. He felt he had to slow down, until a mass of glowing ejecta, a semimolten rock the size of an aircar, plummeted in and lay sizzling within twenty meters of him.

Perry was already in the capsule at the foot of the Umbilical, holding the lower entry port wide open.

Rebka hurled himself through it headfirst, sacrificing dignity for speed. “All right. I’m in. Move it!”

Perry ran madly up the stairs to the control-and-observation chamber, and the car was starting into upward motion before Rebka had picked himself up and checked for injuries. Instead of securing the hatch and following Perry, he turned to the entry port and left it open a foot or so. He peered out.

Whistling lumps of rock and lava continued to pelt the area they had left. He could see fires as the ejecta seared the brush and the dry ground, and hear occasional fragments smacking into the Umbilical above and below them. They would do no damage, unless one entered the open port. He would have time enough to see it coming and to slam the door closed.

The most vulnerable items were the imported aircars. They sat in a neat line at the foot of the Umbilical, built by humans and brought from Opal for local exploration and use. As Rebka watched, a smoking chunk of rock hurtled toward the top of one of them. When it bounced away without making contact, he realized that the cars were sitting beneath a protective sheet of transparent Builder material — cannibalized, probably, from part of Midway Station.

He looked to the horizon. From their present height of two or three hundred meters he could see a long way through Quake’s murky air. The surface was aflame with small flash fires, all the way to the distant peaks. Rising smoke brought a pungent aroma to his nostrils, resinous and aromatic, and the ground below was shimmering with heat and blurred by dust.

It was clear that the source of the disturbance was restricted to the single line of volcanoes that lay between them and Mandel’s glowing face, low to the west. Every third peak carried a dusky plume and a pall of smoke above it. But already the force of the eruption was dwindling. The smoke clouds were no longer shot through with crimson and orange, and fewer rocks came sailing through the air toward the car. The herbivores had disappeared long ago, presumably hiding in the protective depths of the lake. They would know when to come out again.

Perry had left the controls and was crouched at Rebka’s side. The car’s movement up the Umbilical had ceased.

“All right.” Rebka prepared to close the port. “I’m persuaded. I wouldn’t want to take the responsibility for allowing people here at Summertide. Let’s get out of here and head back to Opal.”

But Perry was holding the door open and shaking his head. “I’d like to go back down.”

“Why? Do you want to get killed?”

“Of course not. I want to take a good look at what’s happening, and really understand it.”

“Quake is approaching Summertide, Commander. That’s what’s happening. The volcanoes and earthquakes are starting, just the way you said they would.”

“No. They’re not.” Perry was more contemplative than alarrned. “There’s a mystery here. Remember, I’ve been on Quake before at this time of year, many times. What we just saw is nothing, just a little local fireworks. We should have found more activity than we did, one hell of a lot more. The surface was quiet when we arrived; it should have been shaking all the time. And the eruptions looked impressive, but the ground tremors were nothing. You saw how quickly they died away.” He gestured out of the port. “Look at it now, everything becoming quiet again.”

“I’m no planetary geologist, but that’s just what you would expect.” Rebka could not understand what was going on in Perry’s head. Did the man want people there at Summertide, or did he not? Now that there was a good argument against it, Perry seemed to be changing his mind. “You expect stress buildup and stress release. The internal forces build up for a while, until they reach a critical value, and then they let go. Quiet spells, and violent ones.

“Not here.” Perry finally closed the port. “Not at Summertide. Think of it, Captain. This isn’t normal planetary vulcanism. Opal and Quake revolve around each other every eight hours. Tidal forces from Mandel and Amaranth squeeze and pull their interiors every revolution. At normal Summertide those forces are huge, and the Grand Conjunction makes them even bigger — hundreds of times stronger than they are during the rest of the year.”

He sat down in the lower cargo hold and stared at the wall. After a few moments Rebka went up to the control chamber and restarted their ascent himself. When he came down again, Perry had not moved.

“Come on, snap out of it. I believe you; the tidal forces are strong. But that’s true for Opal as well as Quake.”

“It is.” Perry finally roused himself and stood up. “But the effects are damped on Opal. The ocean surface deforms freely and reaches new high and low tides every four hours. Any seabed changes — seaquakes and eruptions — are damped by the depth of water above them. But the land tides on Quake have no oceans to reduce their effects. At this time of year Quake should be active all the time. It isn’t. So. Where is all that energy going?”

Perry dropped back into his seat and sat there frowning at nothing.

Rebka felt oddly dissatisfied as the upward speed of the car increased and the soft whistle of rapid travel through Quake’s atmosphere began. He had been to Quake and seen evidence for himself. The place seemed fully as dangerous as Perry had warned. And yet Perry himself was not afraid of Quake. Not at all. He wanted to go back there — while an eruption was still in progress!

Rebka reached a conclusion. If he were to understand Perry, he had to have more data. He sat down facing the younger man.

“All right, Commander Perry. So it doesn’t look the way you expected. I can’t judge that. Tell me, then, what does Quake usually look like at this time of year?”

But that was exactly the wrong question. Perry’s look of concentrated thought vanished. An expression of indefinable sadness crept onto his face. Rebka sat waiting for an answer, until he realized after a couple of minutes that he was not going to receive one. Instead of pulling Max Perry out of his reverie, that question had driven him deeper into it. The man was far gone, off in some strange fugue of unhappy memories.

Memories of what? Surely of Quake at Summertide.

Rebka did not speak again. Instead he swore an internal oath, stared up the Umbilical at the distant knot of Midway Station, and admitted an unpleasant truth. He had not wanted this job, a nursemaid task that had interrupted the most challenging project of his career. He had resented being taken away from Paradox, he resented being assigned to Dobelle, he resented Max Perry, and he resented having to worry about the interrupted career of a minor bureaucrat.

But his own pride would not allow him to abandon the job until he knew for certain what had destroyed the man. For Perry was destroyed, even if it did not show on the surface.

One other thing was clear. Whatever had destroyed Perry lived on Quake, close to Summertide.

Which meant that Rebka himself would surely be returning, to a place and to a time where all the evidence proved that humans could not survive.