Hesper Nakagawa (Human, Fliade survey lead)

Nakagawa had slept on it, and in the morning it looked the same.

"From today," said Nakagawa, at the station, before anyone had eaten. "Here is how."

The unit's sound channel at the junction goes off from the moment the one who knocks goes into the third passage, and comes back on when they are back at the junction. For the length of the knock, the party's instrument does not hear the line. The arrivals stay on the channel, as before; those are the party's. The handheld goes with the stone as before, on the chest, pointed low, sound off; what the eye sees is kept. The knock answered is logged by the one who knocked, in their own voice, and that is the record of it, and the only one. Each shift, in the survey lead's own column, the log says the channel was off, by order, and for how long, and the boundary officer countersigns it. And once, today, in the survey lead's hand, the cost: the series the party reread in its second year to find a knock in ends here, at the party's own choice, at a hundred and fifty-six answers since the first stone, and the file says so, so that anyone who reads it afterwards will know it was not a fault.

"It's the one instrument on this station that has never once been wrong," said Egede, "and you're switching it off."

"For the length of a knock."

"That's a hundred and fifty-six times."

"A hundred and fifty-seven," said Nakagawa. "Today's."

Egede made the sound Egede makes when a thing is going to end up in a document, and did not argue, because the objections had all been had, in order, the night before, and were in the file.

Then the junction. The fungus-light, the third passage, the unit on its base. Aravena picked a stone, the size of two hands. Nakagawa knelt to the unit, which was not Nakagawa's job, and told it the channel was off, and said the time, and the unit hummed, once, low, and was quiet.

Aravena went down the third passage with the light held low.

Nakagawa stood at the junction with Egede and the unit and listened to nothing. Not the nothing of two years of evening readings, which had been the unit's word for a thing outside its three classes. The other kind. The crack took the words and kept the shape, and the shape came up the third passage as it had on every shift since the first stone, a rise, going up at the end, and reached the junction, and the unit on its base heard it and had no channel to put it in, and put it nowhere.

Then the wait.

Then the rock. Up through the silt and the floor and the boots, the way it had come a hundred and fifty-six times, felt more than heard, and Nakagawa felt it and knew what it was and did not move, and beside the boots the unit did not move either, and this time that was the same thing.

Aravena came out of the third passage, breath not back, and knelt.

"Log. Shift two-two-oh. Upper junction. Party of three. Knock answered."

The unit hummed. It had heard that. It had heard nothing else, and would not, and Nakagawa stood at the junction and wrote in the survey lead's own column, standing up, which is how this one was always going to be written: Sound channel at the junction off for the knock, by order of the survey lead. Interval stated. The party's instrument did not hear the line. The knock was answered; the party heard it; logged by the one who knocked. Then Ferrant's line, in her hand, when she came down to read it, which she did, that shift, from the edge of the light: Countersigned. Interval as stated. Nothing to grade.

That evening Nakagawa read the unit's channels at the station, the way Nakagawa had every evening for four years. Thermal. Imager: one upright shape on the line, one return, consistent with an adult, a hand from the stone. Sound channel.

Anstolik, passing. Three.

Nothing else. For two years nothing on that channel had meant nothing outside the three classes, and one of the three had been a person. Tonight it meant the rise, and the wait, and the rock, every one of which three people on this roof had heard with their own bodies and could have told you about if asked, and not one of which the file could tell you. Nakagawa read it twice, to be sure that was what it was, and it was.

The log said knock answered, in Aravena's voice. The survey lead's column said the roof had chosen not to know a thing it could have known, and for how long. That was the whole of the survey's record of the one on the line that shift. It was in a person's hand. It could be asked about.

In the morning Aravena would pick a stone, and carry it down, and say the thing, and be answered, and the only things on the planet that would keep any of it were the one who said it and the one it was said to. Nakagawa had been reading the sound channel for nothing every evening for two years and finding it. Tonight was the first time it had been there.