Pattern

Thirty-seven seconds, and then it answered.

The acoustic sensors in the ceiling — the ones Eriksson had run for six weeks of geothermal survey — began producing output on the 18.98 Hz band. Not the blunt broadcast tone of the emitter. Something structured. Nadia held the spectrometer up and watched the waveform scroll across the screen.

It was not random.

She had spent four years analyzing acoustic data before she'd ever worked a disappearance case. She knew what randomness looked like — fractal, self-similar, statistically flat. This was different. The pattern repeated at intervals of approximately eleven seconds. Within each interval, the shape changed. Slightly. Purposefully.

"It's modulating," she said.

Eriksson was on his feet now. Mafi had woken up and was watching from across the space. Paulsen had not moved, but his eyes had tracked to the spectrometer.

"Modulating how?" Eriksson asked.

"Stepped variation. The base interval is constant, the internal shape shifts. That's — that's structured communication." She kept her voice even. She was not going to say the thing she was thinking out loud yet, because saying it out loud would make it real and she needed it to stay theoretical for another few minutes. "Or something that functions like communication. It could be response mimicry."

"Mimicry of what?"

"The three-and-three knock. We heard a pattern. It may be producing patterns because patterns are what we produce." She sat with that for a moment. "Or it understood what I said."

The room was very still.

"How would we know which?" Mafi asked. Her voice was rough with sleep and eleven days of sub-basement air.

Nadia looked at her spectrometer. The eleven-second interval repeated. The internal shape shifted again — different from the previous iteration, different from the one before. Not random variation. Sequential variation. Each one distinct from the last.

She had investigated eleven disappearances.

She had never in her career considered that the question she most needed to answer was whether something was trying to talk.

"We test it," she said. "We produce a pattern. We see if it responds to our pattern or repeats its own."

She picked up the recorder. She tapped the microphone three times, slowly. Paused. Tapped twice.

She played it through the station speakers.

The sensors went quiet.

For four seconds — nothing.

Then: three-and-two. Exactly.

Nadia put the recorder down on the spectrometer case and looked at her hands.

"It's learning the shape of an exchange," she said. "Whatever it is, it understands that communication works in turns."

She had about thirty seconds to process this before the satellite phone on the workbench lit up with an incoming call.

Not from Eriksson's contact number this time.

From a Norwegian Defense Ministry exchange.