Starlight Glimmer raised her head from her Hab cot, ears twitching at a sound she couldn’t hear. Opening her eyes to the most reluctant sliver of wakefulness, she squinted at the Ares III mission clock, giving the time in terms of Martian midnight. The numbers read 05:48. About an hour before local dawn, and more than that before any of them, except possibly Spitfire, would normally get up.

She craned her head to look at Spitfire’s bunk. The pegasus appeared to be sleeping soundly, snoring in a quiet and rhythmic manner. So, whatever had awakened her, it hadn’t been her preparing to launch a pre-dawn safety drill, as she’d done several times in the past.

She considered going back to sleep. Brain, eyes, and body all seconded the motion.

Her bladder, though, exercised a veto, and grumbling, stumbling in the dim light of the Hab during night mode, she tried to make her way as silently as possible to the Hab’s potty. To the relief of all, Mark had discontinued the collection of wastes for compost after the third round of soil experiments demonstrated that the alfalfa was fixing nitrates into the soil well enough to dispense with further soil cultivation.

It was a doubly relieved Starlight who, after a couple of minutes on the oddly proportioned commode, wended her way back to her bunk.

And then she heard the sound which had awakened her; a quiet trickling that had nothing to do with her bladder at all.

The sound was brief, but Starlight had a very good idea where it came from. Waking up quickly, she trotted to the cupboard the ponies had appropriated to store their spacesuits. When she opened the doors, a constellation of green, blue and red lights shone back at her. All five life support systems had been activated.

She was pretty sure they hadn’t been stored that way. Pretty sure. It was too early in the morning for certainties.

She pulled the top suit out- Dragonfly’s, she guessed, since the changeling was in and out of the Hab more than anypony else. The suit legs sloshed. Carefully she tipped out each booted foot, draining almost a pint of water from each leg out through the neckhole.

Starlight was still half asleep. She knew the suit life support systems wouldn’t turn themselves on under any circumstances, but they could be remote-activated from Equestria. She hadn’t quite got around to the conclusion that they wouldn’t do so if nopony was awake to listen unless it was something truly urgent. So, without thinking much about it, she arranged the suit so she could tap on the drinking spigot just inside the neckhole, and began tapping it, sending little squirts of water into the dirt floor next to a cluster of potato plants.

BLM-ESA DE ESA-F54 OP SG WTB K

Translated, politely: “Amicitas calling Baltimare, this is Starlight Glimmer; kindly tell me why have you been filling our spacesuits with water? Over.”

The response came back almost immediately:

ESA-F54 LSS SCRM FLMB GAS DTCTD CVE II LSS SCRM FLMB GAS DTCTD CVE AA INVSTG ASAP K

By the time the message ended Starlight had wet hooves and absolutely zero sleepiness left. She sent back:

BLM-ESA CFM LSS SCRM AA CFM FLMB GAS K

A moment later:

ESA-F54 U HRD ME K

Starlight felt a sudden return of the need for the potty.

BLM-ESA SN STND BY WL UPDT AR

And, as she tapped in the “out” signal, she shouted, “SUIT UP!”

A muddy, sloshy, sleepy minute or two of absolute confusion followed, but at the end of it Starlight had the undivided attention of five wide-awake, grumpy astronauts.

“Home called,” she said, keeping the conversation in English both for Mark’s sake and for practice. “They’ve been calling half the night, going by the water in our suits. The main life support system turned itself off last night. Home says there was a burnable gas detected.”

“Shit!” Mark snapped before anyone else could speak. “Any idea which flammable gas?”

Starlight shrugged. “The only way we know there’s… flambable anything is by the fire on the Equestrian end of the connection. The system shuts down before the flames get hot enough to hurt anything.”

Cherry Berry leaned forward. “Cave on fire?” she gasped, not bothering to struggle for proper grammar.

“Don’t know,” Starlight said. “You know how our life support works.”

“If the cave is on fire, we’re fucked. Bucked,” Mark added in Equestrian. The fact that buck was the one Equestrian word Mark had no problem pronouncing had become an old in-joke among the castaways, but nobody cracked a smile now.

“Okay. Starlight, Dragonfly, get water from out suits,” Cherry said. “Everyone else, quick breakfast. Mark, can tell your home?”

Mark squinted at the clock. “Not for another two hours, I can’t,” he said. “Earth is still below the horizon- hasn’t risen yet.” He shoved the helmet onto his suit- the only one that hadn’t got wet, making him the least soggy person in the Hab- and grabbed a food pack at random from the cabinet. “I’ll go prep Rover 2.”

“Okay,” Cherry said. “We leave fifteen minutes.”

“C-minus,” Starlight muttered to her commander.

“Whaaaaat?”

Dragonfly had taken a few moments during the drying of the spacesuits to inspect the soles of the feet for weaknesses or cracks. The recently repaired surfaces were all good to go, allowing the changeling to gallop alongside Mark’s rover along with Cherry Berry and Spitfire.

Starlight, whose damaged and patched suit couldn’t withstand a gallop, rode in the rover with Mark and Fireball, wishing the whole time she was out on the pre-dawn Martian surface with the other ponies. Mark had thrown caution to the wind, stripping the rover of its saddlebags and extra battery and running across Acidalia Planitia with the electric motors set to maximum. Each time the rover ran into the gullies that criss-crossed the ancient dried seabed, the front end shot forward, then dropped and slammed onto the downslope… and then, on the other side, the front end slammed into the slope, jumped up, and then took flight as Mars gravity proved insufficient to keep all four wheels on the surface. The front end would slam down again only a moment or two after the rear wheels cleared the top of the gully, jarring the occupants yet again.

The rover usually took a little more than half an hour to make the trip from the Hab to the cave farm. Mark’s reckless (and fortunately wreckless) driving shaved that to twenty-four minutes. At the end, as the three staggered out the rover’s air lock to join the ponies and changeling outside, Fireball said in Equestrian, “You know when I said I wanted to drive a General Lee?”

“Yes,” Starlight replied.

“I take it back. If I’d eaten breakfast, I’d be throwing up right now.”

The tiny sun was just rising over the eastern horizon as the castaways trudged up the slope to the cave entrance. As the cargo airlock salvaged from the Amicitas closed behind them, Mark said, “Keep your suits on. If the air isn’t circulating, there could be pockets of solid gas- no oxygen.”

“What kind of gas do you think it is?’ Dragonfly asked.

“It almost has to be methane,” Mark said. “That’s the only flammable gas we’ve ever found here in any quantity. But usually it’s only a trace- like, five parts per billion. There’s almost a million times more water in the Martian atmosphere than methane. But nothing else fits.”

The airlock finished equalizing, and Starlight opened the inner doors. Inside, the cave loomed down and out away from them, the enchanted crystals overhead just beginning to light up as the receiver crystals on top of the hill began absorbing sunlight and magically relaying it into the cave. Nothing was burning; nothing looked charred; in fact, the cave looked pretty much normal.

Cherry was the first out, ignoring Mark’s precautions, hauling off her helmet and then wriggling out of her suit as soon as she could.

“Cherry!” Mark shouted, trying to chase her.

“I can’t check plants in suit!” Cherry shouted back.

“Let her go,” Spitfire said, her suit still firmly on. “I’ll keep an eye on her.” She walked after the commander, as Fireball silently picked up Cherry’s suit and carried it behind her.

“What is methane?” Dragonfly asked after the interruption.

“CH₄,” Mark said. “One atom carbon, four atoms hydrogen. Very flammable. Explosive in confined spaces. The MAV fuel plant makes it.”

“It is poison? Like hydrazine?”

“Not really,” Mark said. “The most dangerous thing about it is that it burns. The second most dangerous thing is, it can suffocate you. It’s not poison, but it’s not oxygen either. A lot of miners died deep underground from methane gas. Both ways.”

“Oooooooh!” Dragonfly’s glowing eyes widened. “You mean… damp-fire? That’s as close as I can get to our word for it. It’s a gas in deep caves and mines. Our diggers used to take…” The changeling glanced at Starlight, then averted her eyes and said carefully, “… things… to tell us when we found some.”

Starlight Glimmer could guess what the “things” were, and it wouldn’t have been canaries. She comforted herself by reminding herself that Dragonfly was speaking of what changelings called the Bad Old Days, Which Are Now Over. And, besides, in her own Bad Old Days Which Are Now Over, she’d done some things almost as bad, so she had no room to judge.

Which didn’t stop her from putting the human between herself and the bug.

The three of them- Mark, Dragonfly, and Starlight- walked over to where the Amicitas’s life support box sat, the main water outlet connected to the hydronic subterranean heating system. All the lights were dark; when the system had shut down, it cut off the hot water too. The cave, which had been comfortably warm just a couple of days before, now felt as chilly and clammy as the first day they began cultivating the soil inside the just-pressurized space.

Mark reached into a pouch on his space suit and brought out a small baggie and a rubber band. He used the band to secure the bag over the air release duct. He brought out a second baggie and rubber band and used those to close off the air intake. “Tell them to turn the system back on,” Mark said. “If it doesn’t work, we might as well go back to the Hab.”

Starlight left that task to Dragonfly, who was definitely the best at using the drinking straw to send Mares code. “Methane,” she said. “One carbon, four hydrogen. You mean like for stoves?”

Starlight could see Mark’s eyebrows go up inside his suit. “You use gas stoves?” he asked.

Starlight shrugged. “Some electric, some gas, some magic,” she said. “In the country, a lot of ponies still use wood stoves.”

“Huh,” Mark said, and muttered in a softer tone, “Learn something new every day.”

The lights on the life support system came on. Both the baggies bulged out; although one side of the air crystal sent and the other received, the two sides weren’t sealed from each other, so the incoming air could pressurize the whole system. The pipes gurgled as fresh hot water poured into the system. Mark breathed a sigh of relief, and Starlight only realized after the fact she’d done the same.

“Okay, we’re still in business,” Mark said. “Now to find out how much methane there is in here.”

“Isn’t any methane bad?” Starlight asked.

“The word of the day,” Mark muttered, pulling out another baggie, “is stoichiometry.” He stepped away from the life support box and into the alfalfa field, continuing, “Stoichiometry is the calculation of balancing out the input and the products from a chemical reaction. In short, it’s the science of how things burn.”

“Stoichiometry,” Starlight pronounced carefully.

“Don’t ask Fireball or Spitfire to say that,” Dragonfly chirped. “They’ll be grumpy for a week.”

“In this particular case,” Mark went on, “methane needs a certain amount of oxygen to burn. The ideal ratio is two oxygen molecules to one methane molecule. Normal Earth air- or your air- is about twenty percent oxygen, or a little more. So for the best burn, methane should be a bit more than nine percent of the air. Too little methane- below six percent of the air- and it can’t burn enough at a time to stay hot enough to keep the fire going. Too much methane- more than about twenty percent- and the oxygen gets crowded out, and the fire suffocates itself.”

“And you’re going to find out how much methane there is using that bag?” Starlight asked.

“Well, I know there’s less than twenty percent,” Mark said. “Because Cherry Berry hasn’t fallen over yet.” Indeed, the earth pony pilot was dashing from one spot to another in the farm, taking a few slow steps here, rushing over there, standing and stepping another moment, then running to another spot. Spitfire and Fireball tailed behind, both obviously expecting a sudden dramatic gasp for air, choking sounds, and a faint, based on their resigned expressions.

“But I’m hoping it’s below six percent,” Mark continued. “Below six percent… well, we’re not safe, because we’ll still be starved for oxygen outside our suits, but we won’t be in immediate danger of a fire or explosion.” The human twitched and added, “That wasn’t fun the first time.”

He handed two baggies to Dragonfly. “Take these to the next two chambers of the cave,” he said. “Do like this,” and he waved another baggie in the air, making it billow outwards, “and then seal it tight.” He held the baggie in his fingertips, closing the airtight zipper shut. “One bag for each chamber. Wait a minute.” He pulled out a marker from his tool belt, made a little mark on his space suit sleeve to test it, and put the letter B on one and C on the other. “B for the next chamber, C for the one beyond that. Can do?”

“Can do!” Dragonfly grinned, fangs gleaming. “Be right back!” With the bags tucked in one foreleg, she galloped off across the immense chamber to the curtain of insulation which hung down over the entrance to the rest of the cave.

As Mark pulled out yet another baggie and walked towards the row of cherry tree saplings along the edge of the farm, Cherry trotted over to them, Spitfire and Fireball in her wake. “Soil is sick,” she said. “Rotting. Worst in center of farm, not so bad at edges.”

Mark grunted. “Yeah, I was afraid of that,” he said. “Spitfire, when did you come here to fly last?”

Spitfire thought about this. “Four, five days?” she guessed. “Day before you came last.”

“Four sols ago,” Mark said. “Notice anything unusual?”

Spitfire shook her head. “Would have said,” she replied.

“Okay. Cherry, would you get back in your suit, please? And Spitfire, could you take a quick fly and see how it feels?”

Spitfire looked at Cherry, and Starlight noticed it took several seconds for the earth pony to realize that the ex-Wonderbolt wanted confirmation of orders. “Oh. Yeah, go ahead,” Cherry said, accepting her space suit back from Fireball and working her way back into it. Meanwhile Spitfire rushed through the same process in reverse.

“Keep an eye on me,” she said to the others as she got her wings free, giving them a test flap. Starlight watched as Spitfire crouched, lifted her wings, and jumped, pounding her wings for all she was worth until she was flying near the crystal-studded ceiling. Slowly, with obvious effort, she made a single loop of the chamber, once or twice swooping down and then climbing back up with intense effort. Finally, a little winded but no worse, she landed where she’d begun.

“Air is…” Spitfire rolled her eyes and switched to Equestrian. “The air doesn’t support me quite like it should. It’s a really small difference, like an altitude change.” She sniffed and added, “And just for a moment, I thought I smelled something rotting, just as I took off. Don’t smell it now.”

Starlight translated this for Mark, who nodded. “Let’s go back to the rover and run these samples through the air testing system,” he said. “But I’m pretty sure I know what it’s gonna tell me.”

Sure enough, the readouts on the rover’s air testing system showed that the samples Mark took around the farm all contained about one percent methane, plus a tiny trace- one or two parts per billion, on the very edge of the equipment’s ability to detect it- of hydrogen sulfide. The two samples Dragonfly took deeper in the cave contained only trace amounts of methane and no hydrogen sulfide.

“Okay,” Mark said, as the Amicitas crew, crammed like sardines in the rover, listened. “I know what’s going on now. Any of you ever heard of methane hydrate?”

Five blank stares answered him. “Not in English,” Dragonfly said for them all.

“Right. Stupid of me. Sorry.” Mark thought a bit and then said, “Sometimes, under the right conditions, ice can freeze in such a way that it traps methane inside the crystals. People have actually been able to burn ice, because what’s burning is the methane trapped inside. And that methane can stay there for millions of years, until the ice melts and releases it.”

“But no ice in cave,” Fireball insisted. “Too warm for ice now.”

Starlight’s heart sank. “Permafrost,” she said, and then added the Equestrian word for it in case the others didn’t know the English one.

“Yeah,” Mark said. “The cave warmed up a lot after Starlight sealed it properly. I’m guessing that heat reached down under the soil to the permafrost inside the cave. And apparently there was a lot of methane in that permafrost. Damn if I know why. But however it got there, it’s been bubbling up through the soil for a while. Guess it’s just now breaking through the surface.

“But that’s not the really bad part.” He pointed to the tiny readout of hydrogen sulfide on the air sampler readout. “Hydrogen sulfide is a lot worse. Unlike methane, it is toxic, and it doesn’t take much. Fortunately, this amount isn’t enough to worry about- it’s less than what you’d make in a fart.”

This failed to get any laughter- to Starlight’s mind deservedly so.

“Thing is,” Mark continued, “you usually don’t find it in a free state on Mars. There’s plenty of sulfur in Martian soil, but hydrogen sulfide breaks down pretty quickly. It reacts with a lot of other things, so it doesn’t stick around too long.” He took a deep breath and added, “Which means it’s being made fresh, and that’s the real bad news.”

“How?” Starlight asked.

“Bacteria,” Mark said. “There are families of soil bacteria that eat methane and fart hydrogen sulfide. And they can kill plants.” He leaned back in the driver’s seat of the rover, bumped heads with Cherry and Dragonfly, and sat up again. “Sorry. I’ve seen it at work before, places where they try to plant decorative plants on top of old landfills. The methane bubbles up through the soil and displaces the oxygen. The bacteria eat the methane and produce hydrogen sulfide gas. The plant roots corrode and rot and die, and the part of the plant that’s above ground usually dies too.”

“You mean the farm is dying??” Cherry almost shrieked the question.

Mark nodded. “You said the soil felt worst in the center of the farm,” he said. “In other words, the warmest soil. That’s where the methane-eating bacteria will have been at work the longest. But it hasn’t been very long since this began. So the plants might recover, but we have to act right now.”

“Kill the bad bachht… baccht… the bad bugs?” Fireball asked.

Mark shook his head. “We’d have to dig up the whole farm to get at them. And they’re not easy to kill. NASA tried to keep them out of the sample soil I brought with me for my experiments. Now we see how well that worked.” He sighed. “No, as much as I hate the idea, I think we’ve got to use the perchlorate spell and get rid of all the methane.”

Starlight would have flinched or flopped on her flanks, had there been room left in the rover cabin to do so. “I don’t think that’s a good idea,” she said. “Gas is a lot harder to handle than dirt.”

“Sludge,” Dragonfly muttered.

“Hot sauce,” Fireball chipped in.

“Mmm,” Mark said. “Remember when we checked the cave for leaks?”

“Yes,” Starlight admitted. “My horn was sore for days after that.”

“You say- you said you okay next day!” Spitfire protested, struggling to keep it in English.

“I lied!” Starlight snapped back. “I wanted to get back to work!”

“I knew it! I knew you lied! Next time-“

“Cut it out, you two!” The shriek was gone from Cherry’s voice; the Ponyville native had shifted back into missile-mare mode. “Go on, Mark.”

“Okay. Instead of two force fields, you have one, near the entrance.” Mark held up two hands to bracket a space in front of him. “Use the perchlorate spell to grab all the methane and hydrogen sulfide inside the cave and put it there, by the airlock.” He paused. “Dragonfly, what does the airlock do with air when it purges it? Pump it back into the cave?”

“No. No point, when we always get more from home. No, it sends it outside, a little at a time.” Dragonfly thought about this and added, “For capsules we pump most of the air into tanks first, so we don’t go off-course when we vent gas. But real airlocks, not.”

“Okay. We have someone inside the airlock, then. Vent the air in the airlock outside. Then cycle air in from the cave- methane and sulfide. And we keep doing that until the bad air is gone- gone for good.”

On paper it seemed like a good plan.

But on paper the perchlorate spell had seemed like a good plan to Starlight, too.

Starlight connected the final cable. The cave had had six of their nine magic batteries in it, slowly recharging. Now Starlight had them all connected in series, ready to deliver all their magic together without her needing to swap batteries mid-spell.

Ideally, one battery, even one-third full as they all were, would be enough. It had been enough for the air-seal check. It had almost been enough for the perchlorate spell, and there had been lots more of that than there was of methane and hydrogen sulfide.

But Starlight did not feel good about this. This felt rushed. She wanted to ask Twilight. She wanted to ask Dr. Kapoor on Earth. She was even willing to ask Discord, even if she knew he’d find the notion of ridding a cave whose soil was more than half horseapples of fart gases too amusing to interfere with.

Come to think of it, had Twilight asked Discord to intercede for them? What had Discord said? Not that the chaos spirit was likely to be able to help in that way- his powers didn’t get along with science in the least. But it should have been an option…

Starlight shook her head, banishing the tangential thoughts, focusing instead on the spell array for the spell she’d used for perchlorates and salt. She concentrated on the molecular structure Mark had shown her- a slightly stilted pyramid shape for methane, a fat two-legged parasprite for hydrogen sulfide.

Cherry Berry, suited up, stood in the airlock, ready to work the controls. Spitfire and Fireball stood outside the cave, watching and waiting in case something went very, very wrong. Dragonfly and Mark stood beside Starlight, Mark’s spacesuited hand on her shoulder, the changeling leaning against her side.

“It’s okay,” Dragonfly said. “You’re the strongest unicorn in Equestria. You’re the mightiest wizard on this planet. You got this.”

Yeah, Starlight thought. I got this.

I got this.

I better got this.

The force field spell came easy, at first, with the batteries pouring magic through her and into the spell. But then came the casting of the material-gathering spell, which was much more complex and energy-intensive than the force field… and the force field was a power hog by itself.

But she cast the second spell, and immediately she knew she’d done it right. Unlike before, when tiny grains of matter could be seen floating up from the ground, she couldn’t see it working. She caught glimpses of wind rustling through the alfalfa, making the leaves of the potato plants shiver, rippling across the cherry saplings. But that was all.

The pressure on the force field shifted. It bulged back, towards her, away from the airlock. She juggled the two spells, keeping the methane and the tiny, tiny fraction of hydrogen sulfide flowing while pouring more power into the force field. The glowing wall of light held steady for a few moments, then began creeping back towards her, forced back by the pressure of the gases on the other side.

Oh.

Oh, buck.

Now Starlight saw the flaw in the plan. The shielded-off section of the cave already had air in it. It wasn’t empty- it was full of nitrogen and oxygen and water vapor and carbon dioxide. And now it was even fuller, as the spell dumped methane and hydrogen sulfide into the small space in a continuous stream that just kept coming and coming and coming.

Starlight tried to slow the methane spell, to constrict it, to meter it in some way. She couldn’t. Too much of her concentration was invested in reinforcing the force field. The methane spell seemed to laugh, dancing away from her, out of her control, drawing power from her willy-nilly. Once begun it would run its course until the magic ran out or until all the bad gases had been gathered in one place.

The force field pushed ever backwards. Starlight forced it to hold firm just before it would have hit the life support box. The glowing wall, unable to proceed evenly, bulged, leaning over the three of them. The top of the wall approached the first of the sunlight crystals, and Starlight had a horrid thought: was the crystal hot enough to ignite the mixed gases? Was there enough oxygen for an ignition?

She was drawing all the power she could from the batteries now; they couldn’t feed mana to her any faster. She tried to talk, but her entire concentration was required to hold the force field in place, to prevent it from crawling one inch more. “Air…” she gasped… “Vent… air…”

She noticed that Mark and Dragonfly were shouting now. She hadn’t heard them. She couldn’t hear them now. Her mind was in the spells. She practically was the spells.

How much power remained in the batteries? She didn’t know. She couldn’t see.

What would happen if the spells tried to draw power when the batteries ran out? Would the batteries shatter, as they had on Amicitas that horrible day?

Would she shatter?

And when would the cave run out of Faust-damned methane??

Something has to give!

And it did.

The spell Starlight had used to permanently seal the cave hadn’t affected the airlock. Oh, the rock had been reformed (again) to an airtight seal around it, but the metal wasn’t bonded in any way to the rock. That part of the cave’s seal, therefore, was the weakest point of the cave.

And, at three atmospheres worth of pressure on one side and one-one-hundredth of an atmosphere on the other, it failed.

Just like with the Hab, the airlock moved. But the cave was made of compressed stone, not canvas, so the airlock didn’t pop loose all at once. As the thin lip of the original cave mouth crumbled away under the sudden eruption of escaping air, the airlock slid forward, slowly, reluctantly, from its seating, grinding at the rock beside and below it. By the time it was completely forced out of the hole, the air pressure had almost equalized. Cherry Berry, inside the airlock, had seven seconds of absolute spacesuit-undergarment-filling terror, but nothing worse.

On the other hand, this now left a gigantic hole in what needed to be a closed environment. Where moments before Starlight’s entire existence had centered on preventing the force field and its contents from backing up into the cave, now it focused on keeping the force field in the cave- and preventing it from crumbling altogether.

Fortunately- oh, thank Faust thank Celestia thank Luna I’ll even thank you Discord- the methane spell concluded at just that moment. Starlight’s mind resurfaced from its magical submergence into reality once more.

“Get that airlock back in the hole now!!” Dragonfly was shouting. “Mark and I can patch it, but we’ve only got seconds! MOVE!!”

“How are we going to do that??” Mark shouted.

“I’m outside the airlock now,” Cherry’s voice came over the suit comms. “Fireball, get over here and help me push!”

“Dirt! Sand and dirt around the edges! I can glue it together!” Suddenly Starlight’s view was full of Dragonfly’s face. The changeling had taken off her helmet. “You have to let the airlock back through the force field, Starlight! Can you do that?”

“How much power left?” she managed to ask.

“It has to be enough,” Dragonfly said. “Can you do it?”

On the other side of the forcefield, which now stood just barely inside the cave itself, and bulged out as it had bulged in before, the airlock crept agonizingly slowly back where it belonged.

Starlight focused, allowed the force field to push a little farther forwards, and then felt it part like gelatin around the inner end of the airlock. There was still a visible gap above the big metal box, and cracks beside and below. Mark, an armful of loose dirt in the doubled-up arms of his space suit, ran towards it, trailing dust all the way. Dragonfly followed, spitting gunk at the sides of the airlock.

That’s not going to work, Starlight thought. The instant I drop the force field the air will blast all of that away.

She looked down, for the first time in a century, at the battery gauges. Almost empty, all six batteries. Again, darn it.

No time left.

This has to work first time.

Starlight took a deep breath- unnecessary, since she was still in her own space suit- and dropped the force field spell. As expected, the goop and the dirt vanished instantly as the cave’s air rushed out the cracks around the airlock. In a few seconds the cave would be airless, and the plants would die- as would Dragonfly, whose helmet was tumbling towards the doors along with a rain of leaves.

But a half-second was all she needed to cast Door What Door, the spell she’d used to seal the cave entrance the first time.

And a second and a half later, the cave was sealed, and the air stilled.

Starlight stepped back from the batteries- staggered back, more truthfully. She felt like a piece of paper, drifting down to earth after having been blown into the air by a cyclone.

Ha, she thought. Buck you, Mars. I am the mightiest sorceress on you. You are nothing before my might.

But the Sorceress Supreme needs a nap just now.

She wobbled, then remembered something. The lights on the life support box had gone out again. Mark’s baggies had blown off somewhere. Slowly, agonizingly, she used her muzzle to tap out a message on her drinking straw:

BLM-ESADE ESA-F54

Mark and Dragonfly stood beside her again, saying something. Starlight couldn’t hear. She continued to tap at the water spigot with her nose.

TRN ON LSS AGN

Was there anything else that needed saying? Oh, yes.

ALL SAFE

There. That should do it.

Mark had picked her up. How nice of him. She didn’t feel like walking back to the Hab just now. She wanted to get back to her bunk and go back to sleep.

AR

And as soon as she finished signaling “out”, she was, out like the lights on the life support box.

Those came back on about ten seconds later, with a couple of interruptions as automatic shutdowns were overridden until the cave was back up to a full atmosphere of pressure.

Starlight Glimmer’s lights remained out for the remainder of the day, not even dreams disturbing a well-earned rest.

Author's Notes:

This scene kept changing, and changing, and changing, and changing. It was changing even today as I was writing it.

Methane was always on the menu for the cave farm, pretty much from the beginning. Now that this part is written, I can tell you that I never intended for the cave to merely breach from ordinary air pressure. The Hab already did that. No, I wanted a boom, and I also wanted something that would threaten the plants even if boom didn't cause a breach. Hence methane.

Scenarios I considered, then discarded:

* Methane hits space heater at right mixture, ignites, sudden air pressure increase cracks cave, breach, cave collapses. (Not believable that anyone inside the cave at the time could survive.)

* Methane quietly smothers the plants and kills them. (Cave would never be left alone long enough to do that.)

* Methane kills the plants directly. (I was surprised as hell to learn methane isn't directly toxic. It only kills plants indirectly, mainly by driving oxygen out from the root system and allowing destructive anaerobic bacteria to flourish. Mark, being a botanist, would know all about this.)

* Methane increases the air pressure in the cave, causes breach. (Again, cave wouldn't be left untended long enough for that to happen. Also, the magic life support system.)

* Starlight uses substance-mining spell, but can't control gases. Methane escapes, mixes with air, hits sunlight crystals, ignites, cave breach. (Somebody mentioned the force field spell from sealing the cave in yesterday's comments, for which I thank whoever it was.)

* As per this chapter, except the forcefield gets shoved far enough back to hit the sun crystals and ignite. (By the time I wrote that far, I realized there wouldn't be enough oxygen left in the confined space for the methane to ignite. Also, Mark, Dragonfly and Starlight were in this scenario too close to the ignition point, with no protection aside from their spacesuits; the concussion would almost certainly kill them.)

And now you kind of know how I got from first plans to how it finally turned out.

And yow, it took a lot longer for me to actually write it than I'd expected, no matter how much I planned it out in my head.

We'll see if I can start doing multiple chapters and start building a new buffer tomorrow, now that the big dramatic lump is past.