Everyone who has gone below B-50 has heard it. Not everyone comes back willing to talk about it, but the ones who do all describe the same thing: a low, continuous hum that does not come from machinery. It is too regular to be geological, too organic to be mechanical, and it responds to the presence of people. Maintenance crews report the hum growing louder when they stop moving — as if something beneath the rock is listening and adjusting its frequency to match their stillness.

The Shelf old-timers call it the Throat. They say the Underworld has a throat, and the hum is it breathing. Scavengers who work the deep tunnels carry earplugs not because the sound is loud — it isn't — but because prolonged exposure produces a sympathetic vibration in the human chest cavity that feels like your heart is trying to match the rhythm. Two tunnel runners in 2197 were found sitting cross-legged on B-54, eyes open, unresponsive, their heartbeats synchronized to the exact frequency of the hum. They recovered after three days of medical care. Neither would discuss what they experienced.

Vossen Utilities has conducted infrasound surveys of the deep tunnels on at least four occasions. Each survey has been classified. The only leaked finding, attributed to a disgruntled acoustic engineer, was a single sentence: 'It is not a resonance. It is a vocalization.' Vossen denies the surveys exist.

The hum has been recorded. The recordings, when played on surface equipment, produce nothing but static. When played on equipment carried into the upper Underworld, they reproduce perfectly. The sound does not leave the tunnels. Whatever is making it, the city above is not meant to hear.