The second surge announced itself with a sound like distant thunder and a pressure that made whiskers lift. Scout was already moving when the water reached the lower passages. This one did not rise as high as the flood that sent them to the upper system, but it hit harder, shorter, like a hammer instead of a tide.
"Strong pulse at the predicted interval," Scout said. "Archive's curve holds."
Archive nodded, then touched the wall as if listening. "The offset is there. Small, but real. After the strong pulse, a lesser one follows. The timing is consistent."
"What does that mean?" Twitchy asked.
"It means the main surge could hide a second threat," Archive said. "If we time our movements to the peak, the echo could catch us while we are repositioning."
"Then we time against the echo," Kai said. "We move early, hold through the peak, and do not shift until the echo passes."
A span of stone that Dig had reinforced weeks earlier failed under the second surge's vibration. The bridge did not collapse, but a crack ran through its heart like a warning. Dig had a repair plan within minutes, but the repair needed bodies, and the echo would arrive before they could finish.
"We have to hold it through the echo," Twitchy said. "If it goes, we lose the fastest route between the upper nests and the medical caches."
"I will hold it," Rend said, already shouldering a brace. Bitey grinned without humor and grabbed another. Dig barked instructions with breathless focus. "Weight evenly. Do not fight the stone, ride it."
The echo hit. The bridge shuddered. Rend dragged breath and did not let go. Bitey braced and did not let go. When the vibration faded, the crack had not widened. The repair could proceed.
Rend laughed once, low and surprised, as if the body had just revealed a secret. "It feels better when the strength means something."
The aquatic predator returned, the first one that had communicated rather than attacked. It stopped at the edge of Kai's territory and released a marker that meant message.
Shadow stood beside Whisper to read. The predator reported movements from deeper territory. Something large had begun to migrate upward. Not alone. Smaller forms traveled with it like escorts or feeders.
"Do you know what it is?" Whisper asked.
The predator released a sequence that felt like weight. Age. Pressure. A thing that changed the water around it just by existing.
"Why tell us?" Twitchy asked.
The predator's body language shifted to something like urgency. It wanted them prepared. It wanted them alive. It wanted allies when the large thing arrived.
"Then we prepare together," Kai said, and sent back a simple marker that meant agreement, and respect.
Tense's verification cycle saved a life for the first time that anyone could prove.
The kit reached a minor entrance and found it ajar by a hair. Tense had closed it hours earlier and had marked the hinge with a scratch to confirm position. Now it was wrong. The kit froze the breath, then signaled quiet alarm.
Inside the space, a thin, segmented crawler waited with stinger raised. It would have killed the first kit through. Instead, Bitey slid a stone slab as a shield, and Striker pinned the crawler from above.
Whisper congratulated Tense the way Whisper knew mattered, with precise, careful words. "Your habit is a tool. The door held because you checked."
Tense blinked rapidly, then checked the next door.
Archive and Whisper worked under the dim glow of bioluminescent moss, stones in a curve that matched the sky's arc as best they could imagine it. ScarMandible's soldiers brought a polished shard that reflected faint light. It was not a mirror the way the ancients might have owned, but it helped.
Archive plotted lunar positions against pulse intervals and discovered a day where the alignment would not only be close but resonant with the cavern's own dimensions. The kit pointed with a trembling claw. "This one. The water will not just rise. It will sing inside the stone."
"What happens when stone sings?" Striker asked.
"It cracks," Dig said. "Or it holds and everything else breaks."
"This is the day we must survive," Twitchy said.
Part Six: Ember and Quick
Ember found Quick scraping shallow grooves in a training corridor, a pattern too regular to be random. "What are you doing?"
Quick breathed fast, not from panic, but from thought racing ahead of breath. "I am laying a run. If the echo hits while we move, I need a path that my body can follow without thinking. The grooves tell my feet where to land."
Ember smiled, warm and proud. "That is what adaptation looks like. You made your speed into a plan."
Quick touched a paw to the first groove. "Will you run it with me?"
"I cannot move that fast," Ember said. "But I can stand at each turn and be a point you can see."
Quick nodded, then ran, and the grooves made a song that was only stone and claws and breath.
Part Seven: The Message in the Depths
Scout returned from a deep sweep with Current at the edge of exhaustion and released a marker that meant urgency and care. They had found a tunnel with walls carved smooth by hands, not water. At the end sat a sealed door, ancient and unbroken.
Whisper brushed the surface and tasted the old markers. "This is not warning, it is instruction."
Archive pressed a paw to the seam and closed eyes. "There is a cavity beyond. Air, not water. The pressure on the other side is stable."
"Can we open it without breaking the balance?" Twitchy asked.
"Not now," Dig said. "The surge cycles make the stone unpredictable. We wait for a lull. We measure. We choose our moment."
Kai stood before the door and let the genetic whisper run through the bones. Up and out had brought them to the ruins. Down and in had brought them here. The ancients had left something. Perhaps a tool. Perhaps a story. Perhaps a mistake.
"We will return," Kai said, and marked the door with a symbol that meant promise.
Part Eight: Onward
They slept in shifts. They ate what they could catch. They trained until muscles shook. They held the bridge. They checked the doors. They counted the pulses. They mapped the echoes. They learned to trust the aquatic predator enough to share signs and to watch each other's borders for a single night.
ScarMandible stood at the edge of the shared corridor and watched Kai as if weighing two plans and one history. Your choices have cost you, the commander said. They have also given you a chance. Few can say that.
Kai did not answer with comfort or with pride. "We will be there when the stone sings."
ScarMandible tilted her scarred head. Then we may both still be here when it is quiet again.
The water in the deep system sighed and pressed its weight into stone. Above, the moons moved toward the place where pull aligns and worlds are tested. In the middle, a colony of imperfect beings kept working, each in their learned way.
They were not ready. No one ever is. They were more ready than they had been yesterday. Sometimes that is the only measure that matters.