The Frequency

Nadia had written her doctoral thesis on infrasound — acoustic frequencies below 20 Hz, inaudible to the human ear but not to the human body. The inner ear registers them as a kind of pressure. The brain does not know what to do with a sound it cannot hear and cannot name.

The effects are well documented. At low amplitudes: unease, a vague sense of being watched. At higher amplitudes: paranoia, disorientation, involuntary dread. At sustained high amplitudes: nausea, panic attacks, visual disturbances. In rare cases, full psychogenic breaks.

The frequency she was hearing, once she ran her portable spectrometer against the ventilation return, was 18.98 Hz.

It had a name in the literature: the fear frequency.

She pulled the station's acoustic logs — every research station this size maintained them for environmental data — and ran a spectral analysis on the eleven days before the disappearance. There it was. Constant, low-level, starting exactly three weeks before the team stopped making contact. For eight days: background noise, easy to ignore, easy to mistake for the building settling. On day nine, the amplitude doubled. On day twelve, it doubled again. By the final three days of log data, it was at a level that would have made the station almost uninhabitable.

She found Paulsen's laptop in a supply cabinet, hidden under two folded thermal suits. There was a partial audio file on the desktop, timestamped six days before the disappearance. She put in her earbuds.

Paulsen's voice, strained and slightly too fast: "It's in the walls. I can feel it everywhere in the station. Mafi thinks I'm imagining it, Eriksson won't talk to me. I can't sleep. I've been awake for — I think it's been forty hours. I think there's someone—"

The file ended.

Nadia sat for a moment. Then she went looking for the source.

The ventilation system was passive. The heating units were standard. She worked through the station methodically, room by room, following the signal on her spectrometer. It got louder as she moved toward the rear of the station's central module.

The generator room.

The door was fitted with a bolt she had never seen on any civilian research station in her career. Military-grade steel, mounted on the outside, secured with a padlock. New enough to still be bright.

Someone had locked it from the outside.