Knock

Nobody moved.

Nadia counted the seconds. The knocks didn't repeat. The outer hull held. She listened to the silence that followed with the specific attention she'd developed over eleven years of investigating things that didn't have good explanations, and what she heard in it was not aggression.

It was patience.

"Tell me everything," she said to Eriksson. "From the beginning. Every field note, every anomaly, every decision. You have about five minutes before I need to start making choices."

He talked fast and she listened faster. The survey had been running for six weeks before the anomaly. The initial response to their broadcast was subtle — instrumentation changes, pressure variations, the kind of data that looks like calibration error until it isn't. Paulsen had been the first to notice the pattern in it. Mafi had tried to reach the mainland when the anomaly started accelerating and found the communications degraded in a way that matched no weather pattern on record. Eriksson had installed the emitter on day nineteen, pulling the design from a classified paper he'd read during his time with a Norwegian defense contractor. It had worked — the anomaly stabilized, retreated to the perimeter, maintained its distance.

Then the infrasound effects had started.

"The emitter overpowered our original signal," Eriksson said. "But it ran at higher amplitude than I anticipated. I didn't account for interior resonance in a metal structure. By day thirty we were all symptomatic and I didn't know how to reduce the output without letting the other frequency through." He looked at her with the expression of a man who had made a series of individually rational decisions that had added up to something he couldn't fix. "We went below to get away from both frequencies. We've been down here since."

"You could have left," Nadia said. "Why didn't you just go?"

He was quiet for a moment.

"Because it had come here because of us," he said. "I didn't know if leaving would disperse it or direct it at the next station. I didn't know what it would do if it followed people across the plateau." He met her eyes. "I thought containing it was the responsible choice."

She looked at him for a long moment. She thought about the three calls to emergency services, all dropped mid-sentence. She thought about the message he'd sent from the generator room, trying to reach her in the four hours between when she'd arrived and when she'd disconnected the emitter.

She thought about what it meant that something had responded to sound.

"It knocked," she said.

Eriksson nodded.

"Three and three," she said. "Not a random pattern. Not structural settling."

"No."

She stood up. She took the portable spectrometer from her pack and the voice recorder she carried for field notes and the basic acoustic equipment she'd brought for standard evidence collection. She laid it out on the floor of the sub-basement and looked at it.

"It responded to a frequency," she said. "Which means it communicates through frequency." She looked at Eriksson. "Have you tried talking back?"

The silence that followed was the specific silence of someone who has been in a hole for eleven days and has not thought of something obvious.

"No," he said.

She picked up the recorder. She adjusted the output to the lowest amplitude that would still carry. She thought about Paulsen's voice in the partial audio file — it's in the walls, I can feel it everywhere. She thought about whatever was standing outside in twenty-below temperatures, knocking in a pattern of three.

She pressed the record button.

"My name is Nadia Chen," she said. "I'm a scientist. I'm not here to harm you. I'd like to understand what you want."

She played the recording through the station's internal speaker system and waited.

For thirty-seven seconds, nothing happened.

Then the station's own acoustic sensors — the ones running the environmental data Eriksson had used for his original survey — began recording an output on the 18.98 Hz band.

Not the broadcast tone from their emitter. Something new. A pattern she hadn't heard before.

Nadia sat cross-legged on the cold floor with her spectrometer in her hands and began to work.

She had investigated eleven disappearances in her career.

This was not going to be the twelfth.