The Gaiss Hollow was wide enough to host a supercontinent, yet still keep spare room. But as far as human scientists could determine, it was subsurface by hundreds of kilometers. The tallest structures ever built amounted to sticks competing with a boulder.

  Beyond the fogged plains that occupied the ground, cavern walls stretched high. Their presence provided reassurance that, despite first impressions, the Gaiss Hollow wasn't infinite. As Pa-5 neared the quaternary checkpoint, she tilted her head back to take in the western cave wall.

  Thousands of stacked humans wouldn't light a candle to the walls' true magnificence. She could see several scars in them, from prospecting expeditions before humanity built the outposts. By that time, emerging scarcity crises had forced humanity to give up on prospecting and exploration for good.

  Ahead of her was a gaping tunnel in the wall. Peering into it gave her shivers, but her legs brought her closer. Her eyes traced it until its roof, which reached a third of the way up the Gaiss Hollow. Making it to the greater western tunnels without running into Aud acted as a tiny victory. Some victory. She couldn't bother to try for a self-reassuring smile.

  The greater tunnels connected the main subterranean space and everything outside it. An entrance to each existed at the center of the four cavern walls. The bastion outposts were forts that guarded and kept a close eye on the entrances.

  Wherever the furthest greater tunnels led, the Aud came from there. No sanctioned expedition or cutting-edge drone had ever made it to the end of the tunnels.

  After a time, the One-Light Directory made the same decision they usually made. They ended all further projects to redistribute resources sorely needed elsewhere.

  The greater tunnels' roofs were high up as well. There was genuine competition between them and the Gaiss Hollow. That was why it was such a surprise when Fort Io wasn't overrun via the ground.

  Unlike countless times before, the Aud's attack vector was the sky. The climb up the walls alone must've taken years, with the median Aud weight accounted for. It must've been even longer to make their way hanging from the cavern ceilings. All the while, their weights and gravity's pull never allowed them to rest.

  She'd pondered her circumstances for a few before realizing she'd gotten rather lucky. Her route for escape led through the western tunnels. Out of all four tunnel systems, the west had been the most thoroughly explored.

  No one could say for certain there weren't missed lesser tunnels or dead ends. She let out a breath of relief while studying the detailed diagram of snaking tunnels that lay ahead.

  The HUD beeped. "Notice: Quaternary checkpoint imminent."

  Stepping across the threshold where the western tunnels began sank her mind back into the familiar adrenaline high. 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.

  It wasn't feeling at home, necessarily, that made her unwilling to enter the north, south, or east. Far from it. As best as she could tell, there were no better alternatives. Once, humanity had seen the western tunnels as a way out to the above world.

  She shook her head. The real question needing to be asked was how many of the tunnels' residents had joined the assault on the fort? It wasn't definitive if Aud behavioral patterns operated on a pack structure.

  Yet the Aud had acted individually during the attack, with each having personal targets. Them existing as a hive mind seemed less likely, being the other predominant hypothesis. Then again, she wasn't a scientist.

  Aud had a strange relationship with each other. They were hierarchical to an extent, with the higher tiers of color taking leadership roles. That usually meant getting the right to take the best kills. Force was a major part in the "proceedings," but the pack leaders--if there were packs, couldn't control every low-tier or dissenting Aud.

  At worst, she should encounter the stragglers that'd stayed, she decided. 'If this is one of those times I'm wrong, won't live long enough to regret it anyway.'

  She checked her drone feeds. The ones she had sent east were out of range, beyond the limits of the WAV's communication module. The connection to those stationed by the fort was fuzzy. She wouldn't have access to those much longer.

  She trusted the programs automating them to guide them back to the capital. Every piece of hardware humanity maintained could go through repurposing. Since there were no manufacturing complexes outside of the Last Light, that was where they'd need to go. 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 deep within the tunnels' bowels, scouting out the many twists and turns. For surprises, which meant threats most of the time. The last drone, the one she'd ordered northwest, was returning. When it arrived, she'd keep it waiting by the greater tunnels' entrance as another warning system, as she did at the valley's edge.

  You could be reading stolen content. Head to Royal Road for the genuine story.

  She continued, the fog gradually dispersing around her legs. She'd never been in the tunnels before, or outside the Gaiss Hollow. She'd seen images, read descriptions, and watched mandatory recordings in the Light Institute. But like everything, experiencing the sights firsthand was off-putting.

  She noticed the increase in gravity before her HUD recorded it. Her steps, which previously felt like marching around glued to metal beams, now felt like limestone chunks had replaced them.

  The HUD was quick to reassure. "Warning: Extended presence in current environment results in reduced performance. Addendum: Projected performance downgrade within acceptable error margins."

  Inhaling took more effort, but what choice aside from trusting the HUD didn't make calculation errors did she have? The only factor that concerned her was the increased pull on her blood. Blood stuck pooling in her legs and feet would slow the amount of oxygen getting to her brain. She'd start feeling the effects at some point.

  Since the fog hadn't seeped into the connecting cavern, she could see the ground. In her hurry, she kicked a large number of stones aside or winced when they crunched underfoot. The second option it was. A few minutes into following the path, one of her drones encountered its first Aud.

  Or rather, pair. Two whites were attempting to scale a wall, clumsily digging into the rock. The mouth of an out-of-reach tunnel looked like their destination. The drone crept closer. It peered beyond them, but they never left its peripherals. Were they trying to make handholds to climb on? But they didn't have hands. Hoof holds, then? Claw holds?

  She flinched when a green emerged from further down the tunnel, shrieking at the whites. "You've got a few kilometers between you and it. It's not that bad."

  Reminding herself of basic facts like that should've soothed her. She just felt like someone was being condescending. The attempt ended up working in a roundabout way, with Pa-5 distracting herself.

  The whites stopped their activity and cowered, backing up against the wall. The green growled, making softer noises. Pa-5's heart thumped. If it got any louder, she'd start hearing echoes of it. She checked to ensure the drone was recording. Clear audio of Aud communicating was rare or spotty. If she could get something...

  She pulled herself up a small cliff, a tight smile on. At least something came out of this situation. She could bring the Directory more than just the report of another failure. Climbing preoccupied her until she heard the clatter of pebbles and a growl. Not from the drone feed. Below her.

  A flash cylinder shot from one of her WAV's armaments. No need to look to aim. The payload fell, tinking and rolling before erupting in a brilliant flash. Its roar masked 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 around to assess; she was off, thumping over and around what she could.

  "Warning: Fur scanner detects yellow. Advisory: Equipped WAV unsuited for confronting tiers above orange. Continue employing retreat."

  Like she needed the HUD to tell her that. Officially, light WAVs could take down white and orange-furs. The hitch was that the process took too long, and every second was fraught with peril. That, or a group of pilots shared the responsibilities and danger.

  Once above orange, the suit's armaments stopped doing meaningful damage. Their protective plating couldn't shield pilots from a single mistake.

  She stumbled over an outcropping, rolling with the suit's knees splayed to the side. The tunnel took a sudden twist down. That forced her to jerk her head back to stop the helmet from banging into a bend.

  She came to a stop next to another cliff face. Realizing her left leg had nothing under it but open air left her cold and clammy. She adjusted, peering over the edge on her hands and knees. The drop wasn't so 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 a panic--although her blood still felt frozen--she could get a clear look at the latest predator out for her blood. It seemed content to study her dirtied and dented WAV in turn, lazily blinking tears from its eyes.

  The head looked like an alligator's, with eyes positioned 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 as dangerous as the rest of its exterior.

  Without all the gore that should stain an Aud's fur, the colored coats themselves were pleasant to look at. She hadn't expected that. This one was a gentle yellow, the shaggy coat clinging to every corner of its body.

  All six of its legs were ready to propel it, poised, even in its relaxed state. The reversed joints had thin enough coverage that she could see the musculature beneath. Tense. A gentle cavern breeze passed by, ruffling the fur.

  Two pinpricks, as bright as a spotlight, met her eyes. She blinked. Could it differentiate where her head was beneath the armor? The tongue pulled back, and it leered.

  Pa-5 made the first move. Launching another cylinder, she rolled the rest of the way off the cliff. As the second grenade went off, she dug the suit's fingers into the cliff wall, slowing her descent enough to tumble upright at the bottom.

  Above her, the yellow charged off the edge, roaring in pain as its ears and eyes suffered twice. It crashed into the adjacent rock wall, bringing down a small collapse. 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 favored straight-line acceleration. The higher tiers could charge 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 going right past. It kept her in sight as it wasted its momentum, taking seconds to completely come to a stop. Reorienting itself and then starting up again didn't take that much longer.

  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 broke in after her. It was wide enough for three of her to stand side by side--out of the suit, that was. She forged through, grunting in disapproval whenever stumbling or clipping the shoulder pauldrons.

  She'd entered the lesser western tunnels. If she equated the greater tunnels to arteries, the lesser ones would be capillaries. No, smaller than capillaries, but the analogy still worked. Here, there was a decent chance she could find one of the tunnels small enough that only she could fit through.