Dean stood outside the classroom, squinting out into the desert. Their destination was a sliver of white against the horizon, contrasted against the black sky. The size of a splinter. The heat haze made it tremble despite the still air.

"How far away do you think it is?" said Emma

"A few days, maybe? Hard to say without knowing how big it is. Hopefully by tonight it will be closer and we'll have a better idea of distance."

"I hope there's something there."

"Even if there's nothing, we've still learnt something." said Dean

"I suppose that's true!" said Emma.

"Did you sleep well?" asked Dean

"I slept awfully." said Emma, "let's head out, if we walk all day we'll sleep like babes tonight."

They each held a plastic bag packed with puckleplum, cactus pieces, their jumpers and a sharp rock knife. Emma had also taken a sewing kit and a lighter. Dean pointed at her bag, "is that a book?"

"Yes! Something to do in the evenings."

Dean checked through his own bag. "I think we'll need to stop often to stock up on cactus, we can drink our fill then, and take a fresh piece with us in case they start thinning out."

"Dean babe, you've checked that bag ten times. You're the sort who arrives at the train station half an hour early I guess?"

"Yes, and I never miss my train." said Dean solemnly.

"Should we take a bearing off that tree or whatever it is? In case we're in a dip and can't see it." said Emma.

"With what? The sun? The stars?"

"We can make a compass, didn't you do that in scouts?" Emma pulled a needle from the sewing kit and stropped it against her bushy hair. She plucked a hair and passed it through the eye of the needle, looped it once around the tip, then held both ends in her hand, dangling the needle horizontally. "This should point north-south." They watched the needle as it hung there completely motionless. "Maybe this is north-south" said Emma, and she shuffled around to point a different direction. The needle didn't move.

"Are you sure that works" said Dean.

"Yes, I've done it tons of times." She held the metal sewing kit box near the needle and it swung slowly around to follow it.

"See? Balls, no magnetic field here then I guess?" she said.

"Let's just be careful to walk in a straight line" said Dean, "and we can leave our spent cactus chunks behind us, in case we need to follow them back."

Dean stopped at the cactus and wiped the sweat that was dripping down into his eyes with his shirt sleeve, serving only to further smear mud across his face. They had been walking for hours and the speck on the horizon hadn't grown. The landscape was unchanged, the same plants, the same cacti, the same density. Maybe this just goes on forever, he thought, this is hell after all.

Emma was stroking the side of the cactus. "Have you noticed how they all seem to have this bumpy bit on them. It always seems to start at the stem, go a little up the body, and be only on one side."

"not really"

"Yes, look" she ran her hand over the bottom of the cactus, where the skin was darker and speckled with bumps, each tipped with a pin-sized hole. She went to an immature one a few paces away, "look. On this one it goes almost all the way up the body, it seems to stop at around shoulder height. Like the cactus grows into it."

"Yeah you're right" said Dean, "it looks to be on the same side, too. Facing the way we came. I wonder if that's the case elsewhere."

"I think so? How do they know the direction? Maybe there is some difference in the light that we can't spot?"

"Maybe the rain comes from a particular direction, and soaks into the cactus through those holes, I imagine it mustn't soak into this rocky ground very well after all." said Dean

"Rain? I guess they must get water somehow. I can't imagine it raining here, though. What would clouds look like in that black sky, with no sun to light them."

At the end of the first day, the speck had become larger. The base was still lost in haze but they could make out the squareness of its tip. From the time it had taken to grow in size, Dean thought that it might be 100 feet tall. They slept beneath a cactus, which gave no shade but a slight sense of security, and continued the next day.

Dean's feet were sore and blistering, the dust and rocks somehow finding their way into his shoe and scraping his feet, already softened by sweat. When they stopped, he would take his shoes off to let his feet dry, squeezing stinging cactus juice to wash off some of the mud. Emma was suffering also, and on the second day decided to cut the toe off her shoes, allowing her swollen toes to stick out and stay dry. Dean was aware that their pace seemed to be slowing. At what point should we just turn back? he wondered. His stomach was knotted from their diet of raw puckleplum and bitter cactus water, it wasn't enough to restore the energy they were spending. Emma didn't complain, so he did the same. He didn't want to sap what little morale she seemed to have gathered these last few days. She remained cheery and joked about her feet as they sat on the dusty ground popping their blisters with the needle from her sewing kit. "I wonder if there are bacteria here, even? Maybe we can't get ill or infected? Nothing would be adapted to our bodies." she wondered.

On the third day the square top of the shape became clear. By the fourth day they could make out details: openings up the side, and they knew it was a made-thing: a tower. On the fifth day, they arrived at its foot.

The dusty ground gave way to a circle of cracked dirt dotted with small dead trees. In its centre, a square grey tower rose ten stories high, Its base brown and stained. A plant that had grown up the side was now dry and dead. At the very top of the tower, a horizontal passage jutted out, ending a few feet out in a mess of mangled metal and glass. There were two large openings other than the passage: a window on the top level and a door at the base. The only other features were four thin vertical slits on each floor on the door side. The inside of the tower beyond the window was hidden in darkness.

Emma and Dean circled the tower, craning their heads. Dean couldn't stop looking at that dark window. It seemed to glow with darkness, the blackness spilling out a little and darkening the edge of the window also. Dean felt one of the dried vine and it crumbled in his hand, "ivy?" he said. He put his hand on the edge of the tower and felt it. "Warm concrete..."

"It must have come from earth" said Emma, "same as us."

"Shall we go in?" said Dean, his eyes fixed on that dark window, "we could sleep here tonight, in the dark, see if there's anything useful inside that we can take back."

Emma stared up at the dark window. "I wonder why it's dark inside... What's different about it to our ruins."

"Dunno, maybe because of the material, or because it was brought here intact, with the roof still on?"

"It feels wrong, like it's some huge dead face staring down at us" said Emma, looking up at it.

"Let's try and peek inside, first"

The door was thick wood, painted blue. The paint was peeling and faded. A small steel grill inserted in the door for ventilation provided a window to the inside. Dean approached, with Emma standing back a few metres. "Be careful" she hissed. Dean put his face up to the grill, cupping his hands around his face uselessly, unable to block the light. "Can you see anything" said Emma. Inside was a flat, featureless black. No shadows or glints of light gave any clue as to what was beyond. A cold air filled Dean's nostrils, bringing the smell of stagnant water and rot, the smell of a dark basement that hasn't been used in many years.

"Hold on, maybe I can use a mirror to send some light inside" said Emma, she rummaged in her bag and pulled out a pocket mirror, flipping it open and angling it back and forth near the grill. The darkness beyond didn't change.

"I don't think that works here. I'll pop my head inside" said Dean, "and have a quick look. Maybe I'll be able to see if I'm inside the darkness."

Emma nodded, her mouth tight. "I guess we've come this far, haven't we. Can't go back now..."

Dean gripped the handle and tried to pull the door open. It didn't budge. There was no keyhole to lock it, only a metal loop to attach a padlock, which was missing. He yanked at the handle and the door shifted a little. "It's swollen" he said, "give me a hand." They both grabbed the handle and Dean put one foot on the wall. They heaved. The door groaned then popped open and they both staggered back. A portion of the darkness inside flowed out, spreading out on the ground beyond the door, pooling like mist, bringing with it that musty, rotten smell. The door hung open at an angle. Its hinges were rusted and the inside edge of the door rotten.

Emma had backed away again when the door came open. "Don't go in, Dean. I don't like it."

"I'll just have a look inside, don't worry." said Dean. He listened for a second, "there's no sound from inside, it's fine." He approached the door, his feet falling into the pool of darkness that was swimming at the entrance, muffling his footsteps. It felt cool and moist after the dryness of the desert. He ran his finger along the rusted hinges on the door, it came away wet and red. "The darkness must have stopped this place drying out as fast as our ruins." he said, "this door is completely rotten though, like it has soaked in water. Maybe the tower only arrived recently? But those vines look long, long dead..." He looked down at his feet, half hidden by the darkness. "I wonder..." he crouched down and emptied some of the things out of his bag, then swung its open mouth through the darkness. The stuff gathered at the bottom of his bag and he lifted it up to the light. It was less concentrated than the inside of the tower, but the inside of the bag now existed in semi-darkness. "How?" he said, he looked back at Emma and laughed. "How?!"

She forced a smile, "ok, let's take some back to the others and go?"

"I don't know how we can transport enough to be useful. We could build a bedroom full of it. I wonder if the smell is from this stuff or something else. I hope it doesn't have to smell like this. Maybe we could air out the tower and move here?" he said. He dipped his hand into the plastic bag, watching the colour drain out of it as it penetrated into the black mist. "It gathered at the bottom, it has mass" he said, "I guess light has mass on earth, maybe here it's the opposite."

"Come on, come on..." said Emma, "I really have a bad feeling. Please"

"Ok, I'll hurry." Dean tied a knot in the bag and set it down outside the door. He picked up his sharp rock from the pile of things he had dumped out. "Just in case" he said, and stepped into the darkness.