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  Mattias nodded. “Good. And why include the migration tunnel? These maps weren't supposed to be historical documents—they're practically blueprints of the original settlement. They were meant to be used. And the tunnel was long reclaimed by the time they were drawn.”

  “And why does this map alone show the sun-sign and the tree-sign?” Thea asked.

  Mattias narrowed his eyes. “The sun-sign is a symbol of the wider world. Tell me again what Lucian said about the tree-sign.”

  “It means something like ‘survival.’ ”

  Mattias had been looking intently at the map while they spoke. Now he turned to her, his green eyes wide. “What if this isn't a map of the settlement at all? What if it's a map of the tunnel?”

  Thea's heart began to pound. “How do you mean?”

  Mattias pointed. “Look here. Anybody knows that the migration tunnel led from the surface to the lakeshore. But this shows exactly where it ended—it's drawn to just where the lake fissure bends near the shore. I know exactly the place it means.”

  Thea nodded. “So do I.” After a minute, she pointed at the map's faded tree-sign. “Lucian said that in the old world, the tree symbol had a literal translation—‘one survives.’ ”

  “One survives,” Mattias repeated numbly.

  “Mattias.” Thea grasped his wrist. “The tunnel is still there.”

  “We're just making guesses,” Mattias said quickly. “We don't know we're right.”

  “Someone is showing us the way. But who?”

  “You're rushing to conclusions, Thea.”

  “Am I? Why was the map left for me, then?”

  “How do I know? Maybe it's someone's idea of a joke.”

  “There's too much effort in it—the paper, the ink. And anyway, it isn't remotely funny. Mattias, we have to find out if it's true.”

  “How are we supposed to do that?”

  “You have some handblowers at the waterwheel, don't you?”

  “I hope you're joking. If Berling ever found out I took a blower….”

  “We'll borrow it late at night and have it back before first light.”

  “No. And I should go now—I have to get to my grandmother's.”

  “Mattias!”

  “What?”

  Thea stared at him.

  Mattias sighed. “Let me think about it,” he said as he started for the door.

  “All right, just think about it,” Thea said, catching up.

  The long spiral of the Mainway was almost empty by the time they neared the old quarter. They were alone as they started around the final arc of the passage.

  “Do you think tomorrow night is a possibility?” Thea kept her voice low.

  “Tomorrow!” Mattias's eyes stayed on the ground in front of him.

  “Why wait? Aren't you excited, Mattias? If we're right about this, it …it changes everything.”

  “I suppose.”

  “You suppose?”

  “What you're suggesting…it might be dangerous, Thea, and not just for us. Have you even thought of that?”

  “But what about the expansion? We’ve talked about it a hundred times.”

  “That was talking. Because you wanted to talk about it. This is different. What if we're right? We should tell someone. Maybe I should talk to Chief Berling.”

  “No. The map was left in secret for a reason. First let's see if the passage is there.”

  Mattias didn't answer, and for the first time Thea wondered: Was she being thoughtless? She only wanted whatMai had wanted. Hadn't her mother risked everything? Hadn't Grace?

  The light was dimmed now; soft green and orange lights burned in globes outside the doors they passed. As they drew closer to the chambers Dexna shared with Rowen, Thea saw that the wooden door was slightly ajar. Orange light spilled onto the pass from inside.

  She heard Rowen's voice, and put a warning arm in front of Mattias.

  Rowen's words became clear as they glided to the open door, their skates nearly silent on the ice.

  “…purpose does it serve?” Rowen asked angrily. “You never have an answer to that, do you? And you fail to accept that it is not your place…”

  Rowen stood just inside the door in her skates and out-erwrap. Seeing Mattias and Thea, she stepped onto the pass and pushed away without another word. She even skated angrily, Thea thought as she watched Rowen disappear around the bend.

  When Thea peeked inside the door, Dexna was standing in the middle of the greatroom, her face flushed. How could anyone argue with her?

  Dexna beckoned them both inside.

  “Thank you,” Thea said quickly, “but I must get home. Lana will worry.” She leaned toward Mattias. “Meet me in the dogs’ shelter after supper. Please.”

  Peter's feet were cold, and the glare of sun on snow made his eyes burn. He watched his father fiddle with some dials on an Automatic Weather Station, which turned out to be a fancy name for a pole stuck in the snow with a bunch of equipment hanging off it-there were two solar panels, a battery, and a bunch of little black boxes that recorded temperature and snowfall and about nine other things.

  The station was put up by someone else years before, and Peter's dad and Jonas had come to make sure ithadn't blown over or been buried by snow. Peter had driven the sled most of the way. Rocketing along with the dogs fanned out in front of him, he was happy. He didn't think about his mother looking sad or his father being distracted all the time. His mind felt empty. He just flew.

  Stopping had been a problem—they overshot the weather station and had to circle back—but his father thumped him on the back and Jonas said he was making real progress.

  Jonas checked the last little black box, wrote some things down in his notebook, and then sighed and grabbed a shovel from the sled.

  “Time to start digging the snow pit. I thought the wind might die down,” he muttered.

  “Want some help?” Peter asked.

  “Nah, we'd only whack each other with the shovels. It's a one-man job.”

  So Peter sat on the sled, rubbed Sasha's belly, and watched Jonas dig a neat square in the snow. With almost every shovelful, the wind blew loose snow back into his face. Jonas grimaced but kept going.

  Peter's mother was off on her own somewhere today-she had already left the geebee geebee when Peter woke up that morning. His dad said she had gone for a walk to think about her book.

  Jonas dug and dug. The pit took shape: It was about a four-foot square, and Jonas was up to his neck in it. Finally, he called Peter's father over to look.

  “Great,” Dr. Solemn said. “Chart it.”

  After all of that work, Peter thought his dad could have come up with something more enthusiastic, but Jonas didn't seem to mind. He smiled at Peter and pulled two Hershey bars out of a coat pocket.

  “All done. Care to celebrate with me?”

  When they had eaten the candy, Jonas went to his pack, pulled out a notebook and a pen, and hopped into the hole. Peter helped by borrowing the measuring tape from his father and holding it at the top edge of the pit. Jonas pulled the tape down to the floor. He pushed his goggles onto his forehead and squinted at the wall of the pit.

  “What are you looking for?” Peter asked.

  “Lots of things,” Jonas said. “I've dug through about a year of snowfall, and first I'm looking for blue streaks of ice—they might show when there was a big storm, compacting the snow into ice, or they might indicate where snow melted and then froze into ice.”

  He began to sketch the wall in his notebook. “I'm also looking at the size of the snow grains, and I'm noting how densely the snow is packed. See, this snow up top is pretty loosely packed—we call that ‘fist’ because I can shove my fist right into it. Then there's ‘four fingers,’‘two fingers,’ ‘one finger,’ ‘pencil,’ and ‘knife.’ Not so technical, right? But it works. Jump down here and I'll show you.”

  Peter slid into the hole.

  “Give it a try.” Jonas pointed to a spot near the bottom. “Jus
t push your fist in.”

  Peter tried. It was like trying to push his fist into a brick wall. He leaned hard, but nothing happened.

  “Try a finger.”

  Nope.

  Jonas laughed. “Hard, right? But try this.” He pulled a regular yellow pencil from his pocket. Peter pushed it against the wall—it went in.

  “Pencil,” Jonas wrote.

  They finished the job together, measuring and pushing and finding streaks of ice that Jonas copied into his book.

  “That was great,” Peter said when they had climbed out.

  “I'm glad you thought so,” Jonas said. “Next time you can dig!”

  “So do those notes tell you anything you didn't already know?”

  “Only when we look at everything together,” Jonas said. “Then we can see patterns: when it snowed, when the temperature rose or dropped, stuff like that.”

  “But why do you care?”

  “You mean, who cares when it snows on a deserted ice cap?” Jonas smiled. “It matters. This ice sheet is about two miles deep, and it's moving.”

  “I don't really get how it moves.”

  “When there's icemelt, water slips down into cracks in the ice. Then whole big chunks slip on the water toward the ocean. Some falls in and melts, and the ocean rises.”

  “That's what Dad is always talking about.”

  Jonas nodded. “It's a pretty big deal. If the whole Greenland ice cap melted, the ocean would be about twenty feet higher, everywhere, even in New York City.”

  “I know,” Peter said. “Dad says that's why we live on the top floor.”

  Jonas laughed. “He's a savvy guy, your dad.”

  Peter drove the sled home, and the dogs stopped on a dime. His mother came in while they were making lunch. She gave Peter a hug, and he felt her shake her head over his shoulder at his dad.

  After lunch, Jonas and Dr. Solemn disappeared into the research tent. Peter's mother sat at the table with her red notebook, and Peter settled into bed with a stack of comic books. After about an hour there was a long sigh from the table.

  He watched his mother stretch and gather herself, as if she were remembering where all of her body parts were located. She put her elbows on the table and looked atPeter on his bed. “Do you have homework to do? I could help you.”

  “Homework?” Peter said. “I'm way ahead already.”

  “Oh.” She considered him. “We should have brought you some radio kits,” she said slowly. “I don't know why we didn't think of that.” He hated the way she was talking, as if she were pushing her words through layers and layers of something—cloth, or fog, or mud.

  “Me neither,” he said.

  They were quiet for a minute.

  “Well,” his mother said, as if that one word cost her untold amounts of energy, “do you feel like learning something about science?”

  “Right now, you mean?”

  “If you want to.”

  “Why not.” Anything to separate her from that red notebook for a while. Anything to wake her up.

  He took a few chocolate eggs from the box next to his bed and joined her at the table, where she closed the notebook and pushed it to one side. Sasha hopped up to follow him and then settled herself again at their feet.

  His mother said, “Do you remember what mitochondria are?”

  “The cell's engines,” Peter droned. He'd been hearing the same definition from her since he was five.

  “Yes,” she said with a half-smile. “I guess it's time we got past that.”

  She seemed to be reviving. Something inside him lifted a little.

  “Mitochondria are like engines,” his mother said, “because they burn energy. But they're engines that build themselves. They come with their own set of instructions— DNA.”

  “I know what DNA is, Mom.”

  “You've learned about nuclear DNA. Nuclear DNA is like instructions to the cells, or a blueprint, explaining how to build the human body. I'm talking about mitochondrial DNA, which tells each cell's engine how to build itself so that the cell can do the work it's supposed to do. Different cells do different work—brain cells do brain work, muscle cells do muscle work. Am I going too fast?”

  She sounded more and more like herself. “No, I'm getting it,” Peter said. “The engines build themselves, and the cells use the engines to burn energy and do their work. Hearing, thinking, moving a muscle, whatever.”

  His mother nodded. “Good. When cells divide to make new cells—it's happening all the time—the mitochondrial DNA, or mtDNA for short, is copied, over and over. Every new cell needs its own mtDNA, so that it can build itself an engine. Still with me? Okay then, here's the tricky part.

  “As the mtDNA—the engine-building instructions—is being copied, changes can occur. Unintended changes,called mutations. And these mutations will change the kind of engine that will be built, just as different sets of instructions will build different radios. Most of the changes are very small, something like coiling the radio wire clockwise instead of counterclockwise around the spool. It shouldn't make a big difference, but sometimes it does.”

  Peter nodded. He'd built enough radios to know.

  “Well, I study those mutations in mtDNA, and how they affect the way people develop. Have you ever heard of Leber's disease?”

  Peter shook his head.

  “Leber's disease impairs vision, and causes blindness in some people. And the disease is caused by one base change to the mtDNA.”

  “Base change?”

  “Think of it as a one-word change in a set of radio-kit instructions. ‘Twist’ instead of ‘rotate,’ something like that. With mtDNA, this one base change makes the cell's engine a little less efficient, a little less powerful. That's fine for some sorts of cells, because some cells don't need a strong engine—skin cells, for example.”

  Peter considered his skin. It didn't seem to be doing much.

  “But other sorts of cells have harder jobs to do. The cells of the optic nerve, for example, need a great deal of energy. So a less powerful engine makes it hard for themto function properly. And that's what Leber's disease is: A mutation in the mtDNA makes the engine a little less efficient, and vision problems occur because the cells of the optic nerve aren't making enough power.”

  Vision problems, Peter thought. Was she trying to tell him something?

  “So,” he said slowly, “if the cell's engine is less efficient, some things won't work as well as they should.”

  “Right. High-energy cells need strong engines—cells of the auditory pathway for hearing, and brain cells, and cells of certain internal organs. And these are all the things we see affected by mtDNA mutations. Muscle tissue also needs a lot of energy. There's something called ragged red disease that results in muscle weakness instead of vision problems.”

  Peter nodded, his toes buried in Sasha's fur under the table. He was flooded for a moment by happiness—his mother was back. She was right there with him. “That's cool, Mom. So you're writing about all of those diseases?”

  She shook her head. “No, those have been written about already. I'm studying other kinds of mutations. I'm looking at what might happen if the mtDNA accidentally improved through mutation, and the cell engines became more efficient, instead of less.”

  “Like extra-powerful engines?”

  “Yes.”

  “Does that ever happen?”

  Mrs. Solemn smiled another real smile. “No one knows for sure. But I think it's possible.”

  Peter glanced at the pad on which his mother had been doodling while she spoke.

  He caught his breath, then jabbed the middle of the page with two fingers. “What's that supposed to be?”

  She looked down at the paper as if she were seeing it for the first time.

  “That's mitochondrial DNA. It's twisted, in a ring.”

  After supper, Thea stroked Peg's ears and waited for Mattias. Before long, he ducked inside the dogs' shelter. “All right,” he said. “Tonight, then
.”

  She sat up. “Really?”

  “Chief Berling has a late meeting with Rowen. I was with him when the messenger came. I'll meet you at the waterwheel. We'll borrow a blower and go to the lake. The lake path is dark after last light; with hope, no one will be anywhere near it.”

  Tonight. She stared at him.

  “Do you want to do this or not?”

  “Yes!”

  “If I don't have the blower back by first light, Berling will never want to see me again.”

  “I know.”

  “Eleven o'clock. Bring a sleigh, and Peg and Gru. The blower is heavy.” He started to leave.

  “Mattias … Thank you,” Thea said. But Mattias said nothing.

  Lana was long asleep when Thea called Peg and Gru, keeping her voice low so she wouldn't wake Lynx, Rowen's companion, or any of the other older Chikchu. Thea crouched next to the dock and harnessed the dogs to her sleigh. She had piled every fur she had onto it, along with a lightglobe and a water sack. They sped along the deserted backways to the waterwheel. She loved being out late, when it was almost dark. The dogs knew the pathways well enough to run with their eyes closed. The thrumming of the waterwheel grew louder as they drew close.

  Mattias was already there, looking grim. He and Ham stood at the edge of the waterwheel's courtyard with a water blower on the ground between them. It was heavy. Together, they loaded it onto the sleigh and covered it with Thea's furs. Neither of them spoke. Mattias harnessed Ham to Thea's sleigh, and they set off for the lake.

  Thea drove the sleigh along the lake path until they saw the deep crack in the shoreline. “Here,” Mattias said.

  He stepped off the sleigh and examined the wall beside them. It looked perfectly smooth. Then he took a special marker from his coat and began to draw.

  “Mattias! Do you think you should?”

  He laughed quietly. “Didn't you ask me to help you blow a hole in this wall?”

  Thea felt silly.

  “Don't worry,” Mattias said. “We use these at the waterwheel. It rubs off.”

  When he was finished, they struggled with the water blower, supporting it together. Thea fought to keep the powerful stream of water within the crude circle Mattias had drawn on the ice wall in front of them. The water ran down the wall and into the lake. A messy hole grew in the wall.