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The passage had narrowed again, and the path beside the stream climbed onto a broad shelf of rock that ran along the wall. It looked solid. It looked like the easiest route she had seen all day, wide and level, with the stream flowing smoothly below it. She was already reaching for it when she saw the stones.

There were three of them, piled one on top of the other at the start of the shelf. The bottom one was big and broad. The middle one was smaller. The top one was a smooth, round pebble the size of her fist. It was a cairn, a tiny, careful tower, and it sat squarely in the middle of the path like a sign.

Her hand froze in midair. Then she pulled it back slowly and took out her hammer. She knelt at the edge of the shelf, as far back as she could reach, and tapped the stone beside the cairn. It didn’t ring. Instead it gave a dull, unhappy thud.

She tapped it again, further along, and heard the same dull, hollow sound. She leaned out and shone her fading lamp along the underside of the shelf, and her stomach turned over. The water had eaten away the rock from below. The shelf was little more than a thin crust, riddled with holes, hanging out over the stream with nothing but habit holding it up. It was the same rotten stone that had collapsed beneath her in the lava tube. If she had stepped onto it, it would have gone down under her, and she would have gone into the stream with it.

She sat back on her heels and looked at the little tower of stones. Somebody had seen this. Somebody had known it wasn’t safe and had known she would come this way, and they had built a warning she couldn’t possibly miss.

“Okay,” she said shakily. “Okay. You’re definitely friendly.”

She found another way around, a narrow scramble along the base of the wall that was harder and slower but solid under her hands. By the time she reached the other side, her head was swimming again and her side ached fiercely, and the lichen on the walls had begun to dim. She found a dry hollow at the base of the wall and stopped for the night.

She ate the other half of her protein bar. She drank from her bottle and switched off her lamp to save what was left of it, and she sat in the soft blue glow and thought.

She didn’t know who they were, or what they were, or why they were helping her instead of simply showing themselves. But they had moved rocks in the dark to make her path safe. They had left her clean water and a cup to drink it with. They had built a warning to keep her from falling.

Her father had always said that you could tell a lot about people by how they treated someone who couldn’t do anything for them. Whoever this was, they had been kind to her, and they hadn’t asked for anything in return.

She dug through her pack until she found her multi-tool. Then she went down to the stream and searched along the gravel until she found a smooth, thin stone, pale grey and about the size of her palm. She sat down with it in her lap and started to scratch.

It took a long time, because the stone was harder than it looked, and the light was dim, and her fingers were cold and clumsy. But when she was finished, the words were clear enough to read.

THANK YOU.

Underneath it, after a moment’s thought, she added a small, lopsided drawing of the leaf cup.

She went back in the dark, slowly, with one hand on the wall, until she reached the little tower of stones. She set her message carefully on top of it and stood there for a moment.

“Thank you,” she said softly. “Whoever you are.”

Then she went back to her hollow and wrapped herself in the thermal blanket and lay down. She thought she would lie awake listening for footsteps, but she was asleep almost before she had closed her eyes.

When she woke, the lichen was glowing brightly again. She got up stiffly and walked back to the cairn. The three stones were still there, stacked neatly one on top of the other.

Her message was gone.

CHAPTER SIX

Korrak had not meant to take the stone. He’d only meant to look at it. He had come back to the little tower in the dark, long after the female had fallen asleep. He had seen her go there with something in her hand, and he wanted to know what it was. He had found the smooth grey stone sitting on top of his warning. He had picked it up and turned it over in his fingers until he found the marks she had scratched into it.

He didn’t understand the first marks. They were straight and sharp, crossing and joining in patterns that meant nothing to him. But beneath them, there was a picture. A small, uneven drawing of a rolled leaf, the same leaf he had left for her on the edge of the basin. She had seen it, and she had used it. And she had carefully left this for him, in the place where she knew he would find it.

He had stood there in the dark for a long time with the stone cupped in his palm. Then he had carried it away with him, back to the warm hollow behind the falls where he slept, and set it on the ledge beside his bed where he could see it when he woke.

Now, in the brightening glow of the moss, he felt guilty about that. She had come back to the tower that morning and found it gone. He had watched from the shadows as she stood there with her hand on the stones and a strange expression on her face. He couldn’t tell if she was frightened or pleased. Perhaps she was both.

She had not stayed there long. She had gone briskly back to her hollow and packed her belongings into her bag, in the same careful order as always. Then she had started downstream again and he had quietly followed.

The passage narrowed as the day went on. The walls drew in on either side, and the stream ran deeper and faster where it was squeezed between them. The path along the bank grew narrower too, until in some places it was hardly more than a ledge of wet rock above the water. The female moved along it slowly, one hand pressed to the wall, and he kept pace with her in the water below.

He had grown steadily bolder over the last few days. He knew she couldn’t see him when he stayed low in the stream, with only his eyes above the surface and the dark water flowing over his back. He knew how to move without a ripple. And she was paying attention to the path in front of her, not the river beside it. Still, twice she had stopped and turned, her fading light sweeping across the water. Twice he had sunk beneath the surface and waited with his heart pounding until the light moved on.

He was almost certain that she knew someone was watching her. She had spoken to him, standing at the top of the climb with her voice shaking a little and her chin lifted high. He hadn’t understood the words, but he had understood that she was speaking to someone she couldn’t see, and that she was brave enough to do it anyway.

He had wanted to answer her. He had wanted to rise out of the shadows and let her see him, and tell her that she didn’t need to be afraid. But he didn’t have the words to tell her anything, and he had seen his reflection in the still pools. He knew what she would see and he knew it would frighten her. So he stayed in the water, and he followed, and he waited.

Late in the day, the passage opened out again into a wide, low chamber where the stream spread across a broad bed of rock. The water was shallow here, rippling over smooth shelves of stone that ran from one side of the chamber to the other. It looked calm and easy to cross, but it was not.