The stone had changed.
Vreth pressed both palms flat against the tunnel wall at Sub-Level 5, deeper than any Pallid had walked in a generation. The rock was smooth here, polished by geological time into a surface that felt almost organic. It was warmer than his skin.
He held still. He let the vibration enter through his hands, travel up the bones of his arms, settle into his sternum. He opened his mouth slightly, unlocking his jaw, letting the resonance climb into his teeth and skull the way his grandmother had taught him before the surface-dwellers came.
The pitch had changed again. In Tunnel 7-South, six days ago, the vibration had been wide and scattered like the sound of a sleeper shifting. Here, deeper, the vibration possessed a distinct rhythm. The pattern felt organic rather than mechanical. A slow, massive oscillation, like the breathing of something so large that each breath took minutes.
