Shayna pulled the door of the airlock tight and turned the lever to initiate lockdown. Her night had been far from restful, but she still felt energized by the knowledge that she now stood on a whole other planet. How had her life gone from theorizing about intergalactic geological origins in front of a laptop in her kitchen to constructing an onsite laboratory one hundred forty million miles away? She punched in her authorization code and waited, blowing a dark, springy curl out of her eyes -- too late to stick it under the tight hood covering her head. At least they had finally gotten the environment working inside the lab. She would be able to take off the clunky spacesuit once she passed through the airlock.

She reveled in such luxuries.

In the first days of their arrival on Mars, Shayna had found herself uncomfortably reliant on the habitat team. Her skills as a geologist would soon be vital as they began to explore the planet and set down roots, but in the beginning, she had been stuck doing grunt work, not that she had minded. Overseen by habitat leaders, she and the rest of the geology team had been tasked with the assembly of their research and habitation pods, just like each of the other research teams. The job had been exciting, though nerve-wracking. Afterall, what if they forgot how to clip the framing together or failed to make a seam airtight? Training seemed to be such a long time ago.

Habitation had kept a watchful eye on all construction and had made sure the landing habitat was functioning at the most basic level regarding air, water, and pressure systems before anyone was allowed to engage in any other kind of work. Each evening, they returned to the control center which had been retrofitted from the ship. In the control center, they escaped the cold and found either comfort or excitement from one another as they reflected on the day's events.

By day three, habitation had cleared them for nonessential activity restricted to the landing zone. The only exception was the bunker team who left early on day three to construct the underground emergency shelter. They began with a ceremony. The captain planted a flag with NASA's emblem, and the highest-ranking crewmembers from each represented country had the honor of planting their own flag. By the end of the ceremony, the ground around the control center resembled a memorial garden that had succumbed to the late summer sun. Dry, desert-like mounds of dirt surrounded each flag post and small stones ringed the display.

Finally, the bunker team loaded up their vehicle and drove off across a dusty, red valley. The research teams were released to begin the work of setting up their laboratories. Marta, the team leader for geology was immediately whisked away to consult on preliminary resource extraction, leaving Shayna to work with Jake to unload their cargo. The first few times, Shayna passed through the airlock, she had been terrified at the idea of removing her space suit and helmet. But the classic astronaut uniform was clunky and difficult to work in.

Watching Jake move comfortably around the unfinished laboratory in a long-sleeved tee-shirt and coveralls – without gasping for breath or suffering damage from improper air pressure – eased Shayna's worries. She quickly grew accustomed to suiting up, making her way to the cargo hold, grabbing the next box of supplies, carting them across the field to her lab, passing through the airlock, and changing into her own set of coveralls before unpacking.

Today, seven days after their arrival, Shayna had nearly skipped all the way from her habitation pod to the geology laboratory. They had finished unpacking the last box of equipment the night before, and she felt light on her feet after days of lugging heavy cargo back and forth. And tonight, would be her first scheduled call home. Tonight, she would see Jeanne's face and find out what ridiculous color her wife had dyed her hair this time. She would ask her children how they liked school on the NASA Families' Campus. She would find a way to make the story of their first days on Mars sound exciting and exotic rather than exhausting and stressful.

So, she bounced on her feet as she waited for the airlock to complete its task. Jake would be running late, as he had each day since their arrival. She could almost hear him calling her from outside the airlock as he jogged over from the living pod. Marta always arrived first. Most likely, she had started inventorying the equipment they had unpacked the night before.

A gaseous puff of air flowed through the vents and Shayna withstood the urge to hurry along and pull the helmet off her head prematurely. Her comm flared to life, sounds of static cutting straight to her eardrums. She jumped. A sharp voice barely recognizable as Jake's melded with the feedback.

"I—run—late," Shayna heard the voice say.

Shayna pressed her comm button to answer, feeling too cheerful to be annoyed by Jake's tardiness. "Yeah, Jake. We all know you're running late. You don't have to tell us if you're only a few minutes behind."

"No—coming."

Shayna rolled her eyes. "I get it. You're coming. Just brush your teeth this time, would y—" A muffled sound like the rolling of old tires on a dusty road came through the walls first, then the comm. What the hell? "Jake? What is that?" She paused and that same sound cut through the comm again. A shriek followed. Shayna's feelings of comfort and good cheer burst, replaced by the sensation of a weighty stone settling inside her chest. A shock of cold followed despite her space suit's temperature control. "Jake! What is that?" A shudder ran through the ground under her feet, and she stumbled. A crash came from inside the lab. "Marta!" Shayna cried. "Marta, are you alright in there?"

The door to the laboratory opened just in time for Shayna to watch a giant boulder crash through the high temperature quartz glass walls. It shattered the glass, tore through the fallen shelves, and took Marta with it. In a moment, her friend was gone. Shayna stood in the doorway; her feet rooted to the floor in icy horror. Her heart beat an unsteady staccato rhythm against the inside of her chest. Without her helmet, Marta would die of an embolism if the impact of the rock hadn't killed her already. Shayna's mouth dropped open in a silent scream.

For an instant, Shayna considered running to Marta. Even if she couldn't help, shouldn't she try? But another shake sent Shayna tumbling through the entrance. As her chest hit the ground, her eyes became level with a bloody skidmark. Fresh waves of terror crashed over her. She couldn't go after Marta. She couldn't look at whatever lay at the end of that trail. Instead, she urged her arms and legs to push her up from the floor, but they were slow to move. She had to run. Escape the projectiles and get out of the building that threatened to collapse. Another shock ran through the ground beneath her. Reflexively, Shayna threw her arms over the bulky helmet that surrounded her head. At least, now, her arms were moving.

When the rumbling paused, and Shayna's heart stopped stuttering, Shayna took a moment to take stock. She was, as yet, still in one piece. She steadied her breathing and tried to focus. What had they told her to do in training? Shayna forced herself to think back many months ago. In her mind, she placed herself in a gray lecture hall, seated in a rickety plastic chair with metal legs. Long, slender tables arranged like stadium seating served as desks. Strips of power outlets ran along the far side of the desks. Shayna plugged in the cord for her laptop as the lecturer paced the front of the room.

Lockdown the habitat. Shayna could see the lecturer's mouth move in the formation of the words. Shakily, she lowered her arms to look around her current environment. Seeing as there were dueling holes in opposing walls, there wasn't much she could do to complete that task.

Assess the situation. In her mind's eye, the words appeared across the front wall of the lecture hall. The instructor liked to type his notes and project them as he spoke. Shayna tried to let the details flow. The ground was shaking, rocks were getting dislodged and knocked through buildings, people were dying. People were dead. Marta was dead. A grim assessment.

Connect to the emergency channel. Chances were, somebody already knew what was happening, and NASA had a contingency plan for just about everything. Again, Shayna brought herself back to the present. Control would be communicating with Earth and disseminating crucial emergency response commands. Shayna took a shaky breath and pressed a button on her comm.

"Control?" Shayna's voice warbled with emotion. "Control, this is Geology. Control, do you read?" In the silence, Shayna prayed for a response and waited as parts of the ceiling began to cave in. At last, she heard another voice.

"Geology, this is Control—"

"Oh, thank God," Shayna rolled her eyes to heaven even as the sounds of tumbling stones heightened.

"We are aware of an environmental event happening in your quadrant. Your orders are to shelter-in-place until otherwise indicated."

In place? Shayna peered around the edge of the airlock. Bits of the ceiling swung dangerously overhead and shorn off lengths of metal protruded from the gaps in the walls like lances made ready for a jousting match. Movement caught her eye through the gap in the wall to her left, and a large boulder wobbled into view. A rumble followed. Angled so that she was mostly out of sight, Shayna watched the boulder rise into the air. Her eyes grew wide, and she leaned farther out of the airlock in horrified curiosity. An appendage made of smaller stones became visible, lifting the boulder. And then—

It dropped.