The sleeping quarters were on the hab-rover's upper level. There were no windows, since there was nothing but gloom and darkness visible outside the steadily trundling vehicle. Instead, there was a large monitor screen mounted on the wall that could show images of the world before The Freeze. The one on the wall of Andrew and Susan's bedroom was currently showing a sunrise above a range of mountains with wispy clouds lit a golden white from below. The sound of birdsong was coming from the speakers.

     The rover was still vibrating gently as the engines powered the six spiky steel wheels, carrying them onward at a steady and sedate ten kilometres per hour, and it rolled gently from side to side as the terrain they were crossing rose and fell in dips and hummocks. Together, they reassured Andrew that nothing bad had happened while he'd been asleep and that they were still on course for their return to the Sellafield dig site early the next day. Nevertheless he still got gently out of bed, being careful not to wake his wife, left the room and descended the ladder through the hole in the floor down to the lower level. His bladder was uncomfortably full, but he had to reassure himself that all was well first. His visit to the bathroom could come later. One quick glimpse into the cockpit was all it would take to settle his mind.

     To his surprise, Jasmine was already there, sitting in the pilot's chair and frowning at the instrument panel on front of her. "Trouble?" said Andrew, suddenly worried, coming forward to look over her shoulder.

     "There was an alert," the girl replied, turning her head to look at him. She reached up a hand to brush a lock of long, auburn hair out of her eyes. "I came to check it out."

     "I didn't hear anything," said Andrew, suddenly concerned. He came forward and dropped into the co-pilot's chair to study the instruments.

     "It was only a level one alert," said Jasmine. "You were probably still deep in the land of dreams. I heard it because I was answering a call of nature. The auropilot says the glacier's shifted."

     Andrew muttered a mild oath. "How much?" he said.

     "Not much, according to the computer. A human, piloting the rover and noticing the discrepancy, probably wouldn't have thougnt it worth mentioning, but the autopilot's a slave to its programming."

     Andrew nodded distractedly while tapping the touch screen in front of him to bring up the terrain data, as measured by the rover's autopilot. He looked out through the cockpit window to see if they were still following their own wheel tracks. Maybe they'd simply drifted a little off course.

     The surface of the glacier was broken into jumbled blocks, all at slightly different levels and angles. After the atmosphere had condensed into liquid nitrogen and oxygen, falling as rain and running in rivers and then freezing solid in the lowest dips and hollows, it had cooled a little more at the surface and contracted, breaking into an almost regular array of slabs like the baked mud of a dried out lake bed. As the relatively warmer and more viscous layers of nitrogen ice below the surface had flowed, slowly but inexorably, these surface slabs had been either pushed together or pulled apart creating a surface that looked as if a careless giant had dropped a huge sheet of glass, that had shattered. The uneven surface of the glacier hasn't worried the Birch family the first time they'd crossed it as the rover was designed to traverse uneven terrain, and Andrew could see the scratches the huge wheels had left in the ice as they had climbed up one sloping surface and down the almost vertical drop of up to a metre to the next slab.

     Then he saw something that made his heart flutter with anxiety, though. Between one tilted slab and the next the wheel scratches were offset by several metres. The offset was a fracture in the glacier where the ice had slipped since the first time they'd crossed it.

     "Professor Bull said this would probably happen," said Jasmine, sensing her father's disquiet and trying to reassure him. "He said it'll probably be centuries before the ice finishes settling into its lowest level, and as it moves, it cracks open..."

     The ground gave a sudden lurch beneath them, startling them both, and a loud bleeping came from the cockpit speakers. "Attention," said the autopilot, its voice calm and reassuring. "An ice shift has occurred. The terrain we are currently traversing may be unstable."

     "Stop," Andrew ordered it. "Stop right here."

     "Acknowledged," the autopilot replied, and father and daughter felt themselves gently pulled forwards in their seats as the rover slowed to a stop.

     Andrew and Jasmine sat, tense and silent, as they waited to see whether the ground beneath them would shift again. "Why didn't it do this on the way out?" asked the girl.

     "We probably cracked the ice without realising it," her father replied. "Or maybe it just slipped on the water ice below all by itself. Something that was going to happen whether we were here or not."

     "What's going on?" came James's voice behind them. He was holding a pair of briefs in one hand and pulled them on while standing in the doorway. Behind him, Susan and David also appeared, roused from their beds by the commotion.

     "The glacier's shifted," Andrew replied. "We may not be able to take the direct route home." He pulled up a map onto one of the monitor screens. "This whole area's almost totally unexplored," he muttered. "South of us was mapped when they surveyed the dig site and north of us when they installed the life hutch, but in between... All we have are radar satellite maps. We know where the hills and valleys are but not much else. We could try going on. Maybe there was just the one small slip and everything's stable from here on." His tone of voice told the others what he thought of the idea.

     "We could backtrack to the north, then go a few dozen klicks east to where the valley's shallower," said James. "There's less of a gradient there so the ice's under less motivation to shift.

     "What about the old maps?" asked David from the back.

     "These are the only maps we have," Andrew replied, a little testily. He immediately regretted his tone and turned to face his son to apologise, but David was already speaking again.

     "I meant, maps from before The Freeze. Maps from when there were still towns and cities out here. And bridges. There might be a bridge from before The freeze we can use. I'd love to see a bridge!"

     "As the ice shifts it takes the bridges with it," said Andrew to David apologetically. "Besides, even the highest bridge would be buried under the ice. It snowed a lot of water snow during The Freeze. There might be five, ten metres of it under us, enough to completely bury everything. We certainly haven't seen any structures from before The Freeze, have we? No trees, no buildings..."

     "Because this used to be farmland," Susan reminded him. "That's why we chose this route, remember? No empty voids in buried buildings getting ready to collapse, creating a sink hole. We avoid towns and cities at all costs, and that's where most of the bridges will have been."

     "This whole conversation may be unnecessary," said Jasmine. "The way ahead may be perfectly traversible. I say one of us puts on a surface suit, goes out and walks ahead of the rover. They can send back a warning if the way ahead is too broken up."

     Andrew opened his mouth to dismiss the idea, but then paused. There was no danger of hidden crevasses. Since the last of the atmosphere had frozen out, there had been no new snow to hide cracks and openings in the ground. Any hazards would be easy to see. He nodded, therefore. "Two people go out," he said. "Tethered together. One walks ahead of the other so that if they fall down a crevice, the other can pull them out."

     "The lighter one goes first, then," said David. "That's means me. I'm the lightest."

     "No, you're too inexperienced," said his father. "We need someone with more time on the surface." His eyes tried to drift towards Jasmine, the next lightest person. He snatched them away guiltily. He wasn't going to put his own daughter into that kind of danger. He'd go out alone, tether himself to the rover.

     Jasmine had other ideas, though, and was already jumping up from the driver's seat. "I'll get suited up," she said, squeezing past her father to get to the door.

     "No, Jasmine!" her father said, though, reaching out a hand to grab her arm. "It's not necessary..."

     "Yes, it is," said the girl, smiling at him. "You know it is. Come on, race you to the airlock!"

     She dashed towards the outfitting room at the back of the rover, and Andrew could only follow her with a sigh of anguish and worry.

☆☆☆

     Most of the differences in level between neighbouring slabs of ice were small enough that Jasmine and Andrew could simply step between them as easily as if they were stepping onto a raised kerb.

     The nitrogen ice was flowing, but very slowly as gravity pulled it at the rate of inches a year towards what had once been the Irish Sea. It moved faster at the centre of the ancient river valley, slower towards the edges, and because it was solid and brittle it fractured as it moved. One day it would reach a stable equilibrium, but that process was only just beginning and the glacier that had once been the river Derwent had a long way to go before it reached that happy state. Andrew and Jasmine came across several places where the tracks left by their previous passage were offset across a newly opened fissure, but after examining each one they decided that the ice had reached a temporary new equilibrium and that it was safe for the rover to cross.

     At one point, Andrew tensed up uncomfortably as he felt the slight tremble of ice shifting beneath him. He stopped in his tracks, and Jasmine was stopped too as the rope between them pulled tight. She looked around with slightly condescending amusement while Andrew waited for the dull, rumbling vibration to pass, and then they continued on, one slow, careful step at a time.

     "So," said Andrew as they continued cauriously on, the hab-rover creeping slowly fifty metres behind them. "You and Joe, eh? Is it serious?"

     "He's just a friend," protested Jasmine indignantly. "Just a friend who happens to be a boy. He's helping me pass chemistry."

     "You spend a lot of time talking to someone who's just helping you pass chemistry," said Andrew. He looked ahead at where his daughter was carefully picking her way across a slab of nitrogen that had tilted as something had shifted beneath it. The cleats on her boots scratched away at the blue ice as she struggled to keep her balance.

     "Like I said, he's a friend," the girl said. "It's not what you clearly think."

     "At your age I would hope not, and he's only one year older than you."

     "Relax, dad. You worry too much." She paused on the edge of the tilted slab, trying to judge whether it posed a hazard for the rover. It felt solid to her, but the rover was a lot heavier. She looked around, the light on her helmet sweeping a spot of light around her feet. It was only a small slab, she saw. The rover would only have one wheel on it at a time. Even if it shifted, it wouldn't bother the large vehicle.

     She continued on, therefore, stepping down the twenty centimetre drop to the next slab with casual ease. "We talk about other things than chemistry," she said.

     "Like what?"

     "The same kind of stuff everyone..." She gave a shocked intake of breath as the slab of ice suddenly tilted under her, causing her feet to slip out from under her with a spray of ice from the cleats of her boots. She fell onto her side and the ice under her immediately began to gush into vapour as the legs of her surface suit, less well insulated than her boots and gloves, allowed her body's heat to escape into the solid nitrogen.

     "Jasmine!" cried Andrew in shock. He started to rush forward but stopped as his training kicked in. One of them had to remain on a secure footing so they could pull the other one to safety, if necessary. If they were both standing in the same place, the same ice shift could get both of them.

     "I'm fine," said Jasmine, scrambling hurriedly back to her feet. She patted her hips and thigh, which were probably numb with cold, in an attempt to goad some life back into them.

     "Is your suit torn?"

     She felt with her hands. "Feels fine," she said. "It's triple weave, it should be okay. Can you see anything?"

     "No," Andrew admitted. "Are you okay to go on?"

     "After a little slip like that? Of course I am. As I was saying, we talk about all kind of things. The TV, sport, pop bands. You know, the stuff us kids like to talk about."

     "He sounds like a good friend," said Andrew, still studying his daughter carefully. He knew her. She might be hurt and not say anything. Not because she didn't want the attention but because she was too stubborn to allow a minor injury to keep her from doing something she wanted to do. She seemed to be walking okay, though, he thought, with no sign of a limp or a bruised muscle.

     "I expect you're looking forward to spending some proper time with him," he added. "Somewhere you can get some proper privacy."

     "We're not having sex, dad."

     Andrew tensed up guiltily. "I wasn't asking," he said hurriedly.

     "Yes you were. I'm still a virgin, you'll be pleased to know, and I'm in no hurry to change that. Not with Joe or anyone else. I'll let you know when it happens. Well, actually, I'll probably tell mum and she'll tell you."

     "I wasn't worried about that," said Andrew, growing increasingly embarrassed. "I trust you. I know you're a sensible girl who won't... Who won't hurry into something like that. I expect you'll want to wait until you're in a serious relationship with someone. Someone with prospects."

      "Joe has prospects."

     "You know what I mean. Joe's a fine lad but he's good with his hands, not his head. He and his dad are the builders of the expedition, not the brains."

     "Don't worry, I'm not thinking about shacking up with some lowly labourer, no matter how great his physical attributes might be. I want to be a scientist one day. The wife of a scientist, with scientist children. It's brains that get me hot and wet, not..."

     "Jasmine!" exclaimed Andrew with shock. "Please don't use language like that. Especially not over an open channel."

     "You should hear what she says when you can't overhear," chuckled David in his earpiece. "When the three of us are doing our homework in our room."

     "Maybe we should stop talking so we can pay attention to what we're doing," Andrew suggested.

     "You started it," pointing out Jasmine. "Asking about my friend who's a boy."

     "Well, now I'm sorry I did. No more talking until we're on the other side of the..."

     "Shit!" said Jasmine, interrupting him.

     Andrew tensed up again and stared ahead. Jasmine seemed to be okay, but she'd stopped and was looking ahead of herself, the round circle of light shed by her helmet torch sweeping from side to side as she turned her head. Andrew waited for her to elaborate, but she said nothing else. "Jasmine?" he said, starting to grow anxious. "What is it?"

     "There's a crevasse," she replied. "A big one. Three metres at least. Way too wide for the rover."

     "Are you okay where you are?"

     "Yes."

     "Okay, I'm coming forward. Don't move."

     Andrew advanced slowly and carefully until he was standing beside her. The crevasse was just as she had described, he saw, going at an angle from behind and to the right to ahead and to the left, cutting across their path as far as they could see. It went all the way through the nitrogen ice down to the intact layer of water ice three metres below, the blue, oxygen rich ice turning to the white of pure nitrogen with depth. The edges were sharp and jagged, with large chunks up to a metre thick separated from the main body by narrow cracks and fissures and seemingly ready to fall away with the slightest disturbance.

     "Our wheel tracks go right across it," said Jasmine. "It wasn't there when we came out."

     "The glacier shifted," said Andrew. "It shifted a lot. I'm guessing it forms a crescent where the entire width of the glacier has slipped downstream."

     "So we might be able to get around it by going a little to the east," suggested Jasmine.

     Andrew shook his head, though. "The ice could shift again at any time. Another crevasse could open up right under us. Sue? You there?"

     "You want us to go back?" his wife replied.

     "Yes. Very slowly, very carefully, but not yet. Jasmine and I will go around the rover and lead the way back out. Keep an eye out for other hazards. Jasmine, let's back away. Okay?"

     "Okay," his daughter replied. Her voice was heavy with understandable fear. The layer of nitrogen ice had clearly not bonded with the water ice below. Maybe the weight of the ice above was creating a layer of liquid nitrogen that was lubricating the transition between the two substances, making it shift more easily.

     "There weren't any shifts the first time!" protested Jasmine, as if complaining that the world wasn't playing fair. " It looked solid, stable. People have crossed plenty of other glaciers. This hasn't happened to anyone else."

     "This is Cumbria," said Andrew. "There's geothermal heat here. Not much. Not enough to make a difference, we thought. Apparently we were wrong."

     "I would say so," Jasmine agreed as she passed the hab-rover and took up position behind it, facing back the way they'd come.

     Andrew positioned himself midway between Jasmine and the rover. "Okay, Sue," he said. "Put her into reverse and back away. Don't try to turn around. Just reverse out."

     "Understood," his wife replied.

     "Okay, Jasmine, let's go."

     His daughter nodded silently and began walking.