Acoustic Resonance

The conduit stretched narrow ahead. A low vibration thrummed through the corrugated floorplates, distinct from water movement. I stopped. Stood still. I leaned forward and pressed the bronze rim of my helmet against a rusted vertical riser pipe. Cold metal against heavy alloy. The hollow resonance traveled straight through the frame—rhythmic, distant clattering. Machinery cycling somewhere deep below. I unslung the tool canister, resting it against the rivet line to steady the reverberation. The frequency settled. Pathway mapped.