He collided with her. Why had he assumed she would need help getting up?

"Run, idiot!" she yelled, and he did run, behind her this time, the only light coming from the distant fliers, the pale glow of which made a halo around the animated shadow she had become.

"How long do you think we need to keep running?" she called back to him.

"Until I can't hear the acid splashing behind me."

"Good plan. Do you think we can outrun it? Is it getting closer?"

"I can't tell. I can't hear it unless I stop."

"Then we might keep running until we drop," she pointed out.

"Good plan," he said.

It didn't seem likely that the glowbirds were flying faster. Yet they were farther ahead than they had been, so he and Robin must be slowing down. His own breath was coming in ragged gasps, and his side was hurting badly. But he had detected no rising of the floor. For all he knew, their present location might actually be lower than the floor in the grotto of Tethys. It was possible that Tethys could flood the entire length of what Chris devoutly hoped was a 300-kilometer tunnel linking Tethys with her sister, Thea. But of course, it was possible the tunnel did not lead to Thea at all. It might end at any moment. It might begin to slope down, and they would find they had been seeking their salvation in what was actually a drain for excess acid. But there was nothing to do but run. If there were an end to the tunnel, Valiha would find it first, and they had not yet caught up with her.

"I think ... it's gone ... up. Don't ... you?"

"Maybe. But how ... far?" Privately Chris did not think they had gained any ground at all, but if thinking they were rising made it easier for Robin to put one foot in front of the other, that was fine with him.

"I can't... do this much ... longer."

Neither can I, he thought. The darkness was nearly complete now. The floor was not as level as it had been, so the danger of falling was increased. Getting up again would be quite a project.

"A little longer," he wheezed.

They bumped into each other, moved away, and hit again. When Chris moved to his right, he hit his shoulder against the invisible tunnel wall. He had his hands out in front of him, no longer able to tell if the glow he was following-seemingly many kilometers ahead-was real or just an afterimage on his retinas. He was afraid the tunnel would make a turn and he would crash into the wall. Then he realized he was moving so slowly by now that he could not be badly hurt in a collision.

"Stop now," he said, and fell to his knees. Robin was somewhere in front of him, gasping and coughing.

For an undetermined time it did not really matter that acid might be creeping along the tunnel behind him. He pressed his cheek to the cool stone floor and let himself go limp. Only his lungs continued to labor, at a steadily decreasing tempo. His throat was burning, and his saliva was thin but so plentiful he had to keep spitting out sticky ropes of it. At last he raised his head, put his palms to the floor, got to his knees, and, by force of will, held his breath for a few seconds, listening. It was no good. His ears thrummed with blood, and Robin, close enough to touch, still gasped and panted loudly. He thought he might hear the approach of the acid if it came in a roaring wave, but it would not. If it were still coming, it would be rising silently. He reached over and touched Robin's shoulder.

"Come on. We'd better get moving again."

She moaned but got up with him. She fumbled for his hand, and they began to walk. His shoulder rubbed the right wall; they continued that way, Chris touching cool solidity with one hand, warm flesh with the other.

"We have to be going up," Robin said finally. "If it was down, the stuff would have washed over us a long time ago."

"I think so, too," Chris said. "But I don't want to bet my life on it. We have to keep going until we can get some light."

They walked on, Chris counting the steps, not really knowing why he was doing it. He supposed it was easier than thinking about what might lie ahead.

After several hundred paces Robin laughed.

"What's funny?"

"I don't know, I ... I guess it just occurred to me... we made it!" She squeezed his hand.

Chris was astonished by her reaction. He was about to point out that they were far from safe, that the road ahead was certainly filled with dangers they could not even guess, when he was suddenly filled with an emotion as powerful as any he had ever experienced. He realized he was grinning.

"Damn. We did, didn't we?" Now they both were laughing. They embraced, slapping each other on the back, shouting incoherent congratulations. He squeezed her hard, unable to stop himself, but she made no objection. And just as suddenly he found himself crying with a smile still on his face. Neither of them could control the swift passage of emotions brought about by the release of unbearable tension. Nothing they said made sense. And in time they were spent, still clinging to each other, still standing, rocking gently and wiping away stray tears.

When Chris finally chuckled again, Robin nudged him, "What's funny now?"

"Oh ... nothing."

"Come on."

For a while he wouldn't say anything, but she kept at him.

"All right. Damn it, I don't know how I can laugh. It isn't funny. A lot of our friends are dead. But back there ... back when we were pinned down..."

"Yeah?"

"Well, you couldn't see this because you were out of it. You know." He hurried on, wishing he'd never started now that he remembered how much she probably wanted to forget that time. "Anyway, Cirocco told us all to pee. Well, hell, I had to. I pulled my pants open and ... you know, got it out ... and let go. Spreading it around, you understand, so it'd do the most good ... and suddenly I thought, Take that, you lousy sand wraiths!" Robin laughed herself to the ragged edge of hysteria. Chris laughed with her but eventually began to worry. It hadn't been that funny, had it?

They had walked a thousand steps before they saw the first glow-bird clinging to the ceiling. It was their first realization that the tunnel had widened around them. The creature was at least twenty meters above, possibly more, and its orange light touched walls that were thirty meters apart. Chris turned and looked for reflections of moisture behind them but found nothing.

In a little while they passed beneath another glowbird, then five in a group. They blazed like torches after so many hours of darkness.

"I wonder what they find to eat down here?" Chris said.

"There must be something. I would think it would take a lot of energy to glow constantly like that."

"Gaby said it was a catalytic reaction," Chris recalled. "But still, they must eat. Maybe we could eat what they eat."

"We're going to need something sooner or later."

Chris was thinking of the supplies still in Valiha's saddlebag. That thought led to Valiha herself. He was beginning to worry about her. By now the glowbirds were plentiful, illuminating a tunnel that stretched far ahead of them. He could see 500 meters ahead, and there was no sign of the Titanide.

"I just thought of something," Robin said.

"What's that?"

"Are you sure this tunnel goes east?"

"What are you-" He stopped walking. "You know as well as I do that..." That what? The stairs had corkscrewed downward for five kilometers. Early in the descent Robin had pointed out that orientation would be critical when they arrived at the bottom. Accordingly, they had performed laborious calculations to discover the rate of curvature of the spiral stairs. When they knew how many steps it took to complete one revolution, once again to be headed in the same direction, orientation became a matter of counting steps. They had determined that they were at the south side of the chamber when they emerged in Tethys, so west would be to the left and east to the right.