A rock had fallen from the sky. When all was said and done, that's how simple it had been. Once upon a time, that had caused the end of the world.

Lysa Sienna leaned against the tunnel's rock wall to take a breather. She squinted toward the entryway and considered the too-bright landscape beyond. All of it had been underwater, once. So had everything. She reflected on the absurdity that the world was cycling back to a similar state.

A trickle of sweat slipped down her brow. Lysa squeezed her eyes shut, wriggling her forehead in an attempt to disengage the bothersome tickle. She was wearing gloves, and the rough leather was covered in grit and grime. Considering her work environment, she didn't trust her hands or sleeves to be clean.

She turned at the sound of gravel-crunching footsteps.

Danielle, a friend and colleague for the dig, gave a wave with one hand while shifting her shouldered burden. She was carrying a sack of mortar, heavy stuff, with the ease of an experience with labor.

Grime covered the younger woman's well-tanned skin, a testament to the length of their project. She'd hardly been a pale pink when they started. "Want me to get that for you?"

"Oh yes, please. It's driving me crazy."

The other woman leaned forward and raised a scarf wrapped around her neck. Her bright green eyes caught the light. She squinted while wiping away the offending sweat and smirked. "I told you to get one of these. Or at least a headband or something. You're just asking for gunk in your eyes."

"I know, I know. I'm just terrible at remembering." Lysa grinned. "But thanks, Dani. How are things going down there?"

"All done, just letting things set." Danielle patted the bag of mortar. "This one's extra, so maybe I'll build that barbecue I was thinking about."

"Cool! Hopefully this means we can restart the bore tomorrow."

That made Danielle wince. "Nawp, sorry. Liam said the inspector left for the day. Said he won't be back until next week. Three-day weekend for him, I think."

Lysa sighed. "Well, shit. That cuts things close. My permit expires on the fifteenth."

"I mean, would he really notice if you dig over the weekend? He only cares about the supports."

"Yeah, but I wouldn't trust him not to check the site logs. If Iker sees fuel reports through the weekend, he'll know we ignored our restriction. I can't afford the fine."

Danielle finally tired of holding the bag of mortar. Sliding it from her shoulder, she set it against the tunnel wall. Bits of the wall flecked away at the touch. The old mine was safe enough, but touching anything made it feel like the walls would fall apart.

"Well." Danielle wiped her own eyes with her scarf. "And maybe I'm crazy for making the suggestion, you could take some time off? Relax?"

"Eh," said Lysa, grumbling. "I should work on the write-up for this week. And send out invitations for next month's workshop."

"Come on, that'll take you what, a few hours? You've got all weekend. Liam and I were gonna head into Prescott for supplies. You could join us, have a decent meal." Danielle grinned. "I know you hate that pre-packaged junk in the trailer."

Lysa teetered on the edge of decision. She didn't think the bore would take much longer to reach its final depth. Setting up monitoring equipment would take longer, but this would be the sixth they'd installed during the trip. They had practice, and were an efficient crew.

Plus, she could finish most of the paperwork during the trip home. The drive would take several days, and her overnight stops would be perfect for catching up on documentation. "I guess I could do it on the road."

"Oh yeah, plenty of time there. Here back to Colby, ugh, that's five days? Four if you push yourself?"

"Yeah." Lysa rolled her eyes. As if on cue, her stomach chose that moment to growl. Betrayed by her own body. She shrugged. "Okay, fine, you convinced me."

"Yikes," said Danielle. "Don't sound so excited."

She smirked at her friend. "I am trying." She gestured toward the bag of mortar. "Need help with that?"

"Psh, girl. Get your ass down to the dig and shut everything down. And tell Liam to hurry with those measurements. I heard your belly rumbling."

"Okay, but don't say I didn't offer." Lysa turned away and turned on her headlamp. Back into the depths.

#

Drilling could be dangerous work, and it was usually expensive and hard to protect. Do you buy a protective environment and seal away the dig site? How do you power your drill? What happens to the entry point after you leave? Does it get filled in so the next crew has to start from scratch?

An abandoned mine could be a perfect solution to several of those problems. If previous owners had kept to the bare-minimum of safety standards, you had a pre-built environment for geological research.

Plus, there were existing power lines running into the tunnels, and several of the mine junctions had pre-installed lighting. The dirty, half-working illumination was spotty at best, but it was better than working in the dark.

But best of all, a mine meant they didn't have to dig as far into the earth. That meant less money and less time.

Lysa had managed to secure long-term research permits for six abandoned sites. Well, two were partially-abandoned, but the owners hadn't cared about the dead-end shafts and were happy to get some return for their unrewarded hopes.

That had taken longer than the permits. Funding. Sixty-some thousand dollars had been allocated for the entire trip, and eighty percent had gone to the mine owners. Or local governments. Or taxes.

That had left little more than ten grand to fund equipment purchases, travel, and some form of lodging through the six-month trip.

Lysa was tired, and she was nearly broke, but she had accomplished everything they'd set out to do. With any luck, the results would make for a solid paper on modern data gathering in critical zones. She'd certainly been impressed with the combined data from new techniques in RADAR, LIDAR, and sample analysis.

Plus, long term they had new seismology stations added into the temporary network of sensors. With any luck, they'd prove reliable enough to serve as reference sources for the national seismic system. Lots of successes, lots of data to crunch, and good experiences with her colleagues.

It had been a great trip, and she was sad to near its end.

Her thoughts melted into a quiet state of calm as she continued through the mine. It wasn't a short trip, the hike down to the boring equipment, but it was easy enough. The site's original owners had put in rail and kept the exploratory shafts at an easy grade for walking.

There was a lot of quiet along the way. And cold. It was noticeably chilly after the first hundred meters. She enjoyed the escape from Arizona's heat.

Rounding a final corner, she smiled as she saw Liam at work in the main cavern. The lights were better in the wide junction, and the ceiling was a meter higher than elsewhere. No looming threat of bumping your head. Lysa pulled her headlamp down onto her neck and waved.

Liam didn't see her, and in fact seemed entirely oblivious to anything except his work. He had the LASER rangefinders in a new configuration since last time. His darks eyes were narrowed in a concentrated squint behind heavily-rimmed glasses. The man claimed to have great vision, but you wouldn't guess so by the way he leaned into every moment of observation. It was like watching Mr. Magoo peering at unfamiliar surroundings.

"Liam, you almost done?"

He didn't hear her, of course, because Liam was also wearing his headphones. He was always wearing headphones

Getting closer, Lysa could hear the tinny whisper of music. She waved in another attempt at catching his attention, but had already given up. She knew how much it sucked to be surprised after being alone in the dark caves. That, and she didn't want to disturb him while he crouched over the system panel.

Accurate measurements were critical to getting the sensors accepted for future research and reference inclusion.

So, seeing no reason to hurry, Lysa crossed her arms and watched. Liam was an expert with the measurement equipment, and seeing his process was a helpful reminder on procedure. He made an adjustment, then stooped over the readout, adjusted, stooped.

A flicker of color caught her attention. Across the room, a status light went green to amber. That was odd. Circling around Liam's side, Lysa walked to the boring terminal.

They had already installed the seismic monitor. As part of their tests, they wanted accurate readouts on the boring unit's seismic signature.

The display showed a warning light.

Lysa crouched and tapped the panel. She flipped through menu screens and went to the error logs.

'OOS: Tolerance failure.'

"Out of synch?" she asked herself. "What the hell does that mean?" Sighing, she pulled out her phone and scrolled to the operating manual she'd downloaded.

She peered at a section titled, "Troubleshooting Steps."

'OOS: Tolerance failure - The system is no longer synchronized with some or all of its external sensors. OOS faults usually indicate a significant increase in signal noise. Possible causes include improper grounding, loose sensor mounting, or faulty devices.'

Lysa stood up with a grimace. "Well, shit. And we just got this thing calibrated.

A hand clasped around her shoulder.

She jumped to the side with a shriek.

Liam stepped away with raised hands. "Sorry, shit! Sorry!"

"Fucking, fuck, Liam! You scared the piss out of me!"

He cringed. "Hopefully not literally. Sorry again."

Lysa put a hand over her heart and let out a long breath. "Gods, you asshole. And I just did all I could to keep from surprising you while you got your readings!"

"Oh, you've been here that long? Huh." He glanced toward the rangefinder array. "Yeah, those things are being a pain in the ass." His voice withered in volume. He was distracted.

"Huh. What do you mean? They've been working alright so far."

"Yeah, but it's like they're busted somehow."

Lysa winced. "Dammit. That's the last thing we need. Those are loaners. I can't give them back broken."

"Well, that's what's strange." Liam shrugged. "They don't really act like they're broken." He scrubbed a hand through his frizzy kinked hair. "Just. The readings aren't right. Maybe the sensors got dust in them?"

"Huh, I guess that's possible." Lysa glanced toward the seismometer. "The seismometer is acting up too, but I thought we kept everything sealed during the installation."

Her stomach suddenly dropped. A feeling like driving too fast over a dip in the road.

Liam shivered. "Whoa, that was weird."

"You felt that too? What just happened?" asked Lysa.

They felt it again.

And then, everything began to move. There was a terrible grumble that they could feel. The sound surrounded them, coming from every direction at once.

The lights went out.

Lysa shrieked out of some primal, pent-up fear beyond consciousness.

"Oh shit, oh shit, oh shit," came Liam's voice, and it grew distant and whispered and hollow with distance.

Something cracked, and the grumble rose in volume until it was a vibration and then a terrifying shaking of the surrounding rock. Nothing so solid was supposed to move so quickly.

"Liam? Where are you going? Where are you!?" Lysa held her hands in front of her, feeling, groping for something to hold onto, searching.

The ground itself tilted, or maybe she tripped on something, but the dark was too absolute to tell. Too late, she remembered the headlamp dangling around her neck.

Too late as she fell and tumbled across the quaking stone floor.

Something hit her, sudden and firm and pleasantly cool. She felt a stinging slap on the side of her head, saw too-bright stars, and then blacked out.