The space humanity had called home for more than three centuries defied conventional science. The Gaiss Hollow was wide enough to host a supercontinent within its belly and still have room to spare, but as far as human scientists could determine, it was still hundreds of kilometers beneath the surface.
Beyond the fogland plains that occupied most of the ground, cavern walls stretching higher than anything humanity could build walled in the space, providing reassurance that despite first impressions, the Gaiss Hollow wasn't an infinite space. As Pa-5 neared the quaternary checkpoint, she tilted her head back to take in the Western Cave Wall.
Something told her thousands of humans stacked on each other wouldn't light a candle to the walls' true height and magnificence. She could see several scars in the walls, evidence of climbing and drilling expeditions over the years before humanity was pushed back to their outposts, and by then the emerging scarcity crisis had forced humanity to give up on prospecting and exploration missions.
Ahead of her was a gaping tunnel in the wall, maybe reaching a third of the way up. The Greater Western Tunnel System was the first checkpoint in her journey.
Located at each of the four ends of the Gaiss Hollow's walls, the Greater Tunnel Systems acted as connectors between the main subterranean space and everything beyond, above, and below. Each fort had originally been constructed as both watchtowers and guard posts to keep a close eye on the tunnel systems when it was discovered ages ago that the Aud came from these tunnels. No sanctioned explorer or cutting-edge drone had ever made it to the end of the tunnels, and after a time, the One-Light Directory made the same decision they usually made and ended all further projects to redistribute resources needed elsewhere.
The Greater Tunnel Systems' cavern roofs were impossibly high up as well, putting up a genuine competition between them and the Gaiss Hollow. That was why it was such a surprise when Fort Io wasn't attacked from the ground, like countless times before, but the sky. The climb up the walls alone must have taken years, with how heavy even the lightest Aud was. And from there, it must've been even longer for them to make their way hanging from the cavern ceilings, all while their combined mass and gravity never gave them a rest.
It was a lucky series of circumstances, in a way, that she was positioned at Fort Io and then given the route to retreat through the Western Greater Tunnel System. Out of all of the four major tunnel systems, it had been the most thoroughly explored. No one could say for certain there weren't hidden crevices that lead to the minor tunnel systems, or further to another of the major tunnel systems, but as Pa-5 pulled up a separate map, it relieved her to see a complex and detailed diagram of the snaking tunnels that lay ahead.
The HUD beeped. "Confirmation: Arrival at quaternary checkpoint imminent."
As she stepped across the threshold separating the Gaiss Hollow from the tunnel system, her mind sank into high gear once more. Should she take her time here, or charge onward? She'd be hesitant to commit to the latter plan if her route took her through any of the other greater tunnel systems, but as mentioned, the western tunnels were the apex of what humanity had achieved in their prospecting efforts. She remembered how academics back in the schools of the capitol crowed that, once, humanity had seen the Western Greater Tunnel System as a way out to the world above.
She shook her head. The real question she needed to ponder was how likely it was that most of the immediate residents of the tunnels had been recruited to assault the fort. The leading scientists in charge of studying Aud behavioral patterns weren't aware of if they operated on a pack system or hive mind structure, but from how scattered the Aud had appeared just a few minutes into the attack, with each having their targets or interests, she would caution to assume the latter wasn't as likely.
Aud had a strange relationship with each other. Their rudimentary relations were certainly hierarchical to a degree, with the higher-tier colored ones taking roles of leadership, or getting the right to take the best kills. Force was a major part in the decision of who ruled, but the pack leaders--if there were packs, couldn't keep every low-tier or dissenting Aud under their purview.
At worst, she should expect to encounter the stragglers that had been left behind, she decided. She accessed her drone feeds again. The ones she had sent east were out of range, beyond the limits of her WAV's transmission capabilities. The connection to the ones stationed by the fort was fuzzy, and soon on their way out as well. She trusted the programs responsible for their automation to guide them back to the capitol before long. Every single piece of hardware humanity maintained could be used for some task. This mentality of reusing and recycling had become critical in the fight to stay afloat.
Total war was costly, after all.
The ones she'd sent ahead of her were already deep within the bowels of the tunnels, scouting out the many twists and turns for threats. The last drone, the one she'd ordered northwest, was now on its way back. When it arrived, she'd have it waiting by the connecting cavern to serve as another warning system, as she did at the valley's edge. She continued, noting the sudden disappearance of the fog.
She'd never been in one of the tunnel systems before, or even in a location that was external to the Gaiss Hollow. She had seen images, read descriptions, and watched recordings like all the others, but like everything, it was a different experience to see and feel the differences with her eyes.
Her HUD recorded a moderate increase in gravitational force, something she noticed before it did. Her steps, which previously felt like marching around wearing a tree, now felt like limestone blocks were glued to her soles. The HUD was quick to reassure her, informing her that only a minor performance loss would occur if she stayed in this environment for an extended amount of time. It took a bit more effort to suck in air, but she was confident the HUD hadn't made an error in its calculations.
The only factor that concerned her was the increased pull on her blood. If her body couldn't properly send blood to where it was needed since it'd be pooling in her feet and legs, eventually she'd be affected by the lack of oxygen arriving in her brain. So, even if she was already operating on a timer, now there was a second one over her head.
Since the fog hadn't seeped into the connecting cavern, she could see every stone she kicked aside or crunched underfoot in her hurry. The second option it was. A few minutes following the path, one of her drones encountered its first Aud. Or rather, two of them. A pair of whites were attempting to scale a wall separating them from a higher tunnel, clumsily digging into the rock. The drone was directed closer, half looking beyond them but still keeping them in the peripherals. Were they trying to make handholds to climb on? But they didn't have hands. Hoof holds, then?
A shriek sounded further down the tunnel. A green emerged from the darkness, shrieking at the whites. They stopped their activity and cowered, backing up against the wall as the green approached. It growled, making softer noises. Pa-5's heart thumped with the energy of a star. She checked to ensure the drone was recording. Although it was common knowledge that the Aud could communicate, clear audio of the phenomenon was rare.
She pulled herself up a small cliff, a small smile on. At least something came out of this situation. She was preoccupied until she heard the clatter of pebbles and a growl. Not from the drone feed. Below her.
Without looking, she shot out a cylinder from one of her WAV's armaments. The flash grenade fell, tinking and rolling before erupting in a brilliant flash, its roar masking the Aud's scream of pain. Even protected in the suit, Pa-5 felt the heat from the flash and her ears buzzed. She didn't turn to assess the circumstances, she was off, thumping over and around what she could. Her HUD flashed red.
"Caution: Presence of yellow fur is detected. Recommendation: Light WAVs are unsuited to confront tiers above orange. Continue employing retreat."
Like she needed the HUD to tell her that. Officially, light WAVs could take down white and orange-furred WAVs, but the process of doing so either took an incredibly long time and was fraught with peril, or the responsibilities and danger were shared among a group. Once the tiers got higher than orange, the suit's armaments stopped doing meaningful damage and their protective plating could no longer shield pilots from a mistake.
She stumbled over a small boulder, crashing and rolling on the suit's knees as the tunnel took a sudden twist down. She rolled and rolled, coming to a stop next to another cliff face. She peered over the edge on her hands and knees, seeing the drop wasn't far. Looking back, at the peak of the tunnel she spotted the Aud.
Powerful. That was the first word that came to mind. Now that she wasn't caught up in as great of a panic as she was during the fort's assault--although her blood still froze into something foul, she could get a good look at the latest predator out for her blood. It seemed content to study her dirtied and dented WAV as well, lazily blinking tears from its eyes.
The head was angled like an alligator, eyes positioned at opposite spots further back in the skull. It's maw peeled apart, lipless, and dripping saliva. A thick, fleshy tongue probed the air, twitching and dancing. She didn't doubt it was just as hard as the rest of its exterior.
What surprised her was that without all the gore and blood she normally saw staining an Aud's fur, the colors and fur composition themselves were pleasing to look at. This one was a gentle yellow, a shaggy coat clinging to every square decimeter of its body.
All six of its legs were poised and ready to propel it, even in its relaxed state. The reversed, animalistic joints had thin enough coverage that she could see the musculature beneath, tense and begging to be released. A gentle cavern breeze passed through, ruffling the fur.
Two pinpricks of light, as bright as a spotlight, met her eyes. She blinked. Could it differentiate where her head was beneath the armor? It snorted.
Pa-5 made the first move. Launching another cylinder, she rolled the rest of the way off the cliff. As she fell and the second grenade went off, she dug the suit into the cave wall, slowing her descent just enough to tumble on her feet.
Above her, the Aud charged off the edge, roaring in pain as its ears were assaulted a second time. It crashed into the adjacent rock wall, bringing down a small collapse with it. While running, she felt the thuds beneath her feet. She knew she couldn't outrun it, and it wouldn't take long to get its legs under it and give chase.
Aud were beasts that favored straight-line acceleration, the higher tiers capable of charging past humanity's fastest transportation with ease. So she didn't try to beat it in a race. She just needed to reach the first branching tunnel, and then--!
She jumped to the side, the Aud screeching right past. It kept her in sight as it pulled the breaks, taking just seconds to completely come to a stop, reorient itself, and then start up again. She entered another tunnel, this one the smallest by far. She'd gone off the path, and the HUD struggled to find another route while the Aud struggled in after her. It was just wide enough for maybe three of her to stand side by side, she forged through, grunting in disapproval whenever she stumbled.
She'd entered the Lesser Western Tunnel System. If one imagined the greater tunnel systems as the arteries and veins surrounding a heart, then the lesser tunnels would be the size of capillaries, no, smaller than capillaries. Here, there was a decent chance she could find one of the tunnels small enough that only she could fit through.