Yuna draws the grid with a grease pencil on a plastic sheet over the chart, the old way, because screens lie when you poke them in a hurry. Each square is a promise: ping, listen, log, move the aim a little, do not fall in love with the first bright thing.

“Polite power,” she says. “Enough to hear rock and metal. Not enough to paint the whole canyon as one scream.”

You sit in the second headset. The ocean in the phones is not silent. It is a fizz of shrimp somewhere up-slope, a groan of cooling rock, the habitat’s own pumps pretending to be important. A ping is a tap you choose. The return is a tap you measure.

TIDE overlays travel times. One return is fat and near: the terrace wall you already knew. One is thin and farther, in the vent bearing. Thin can mean small. Small can mean Sprat. Small can also mean a chimney the size of a bus if the ray bent.

Vargas leans in the door, fleece zipped to the chin. “Grid or glitter, Aquanaut. I will not pick for you if you can still do it.”

Reed on comm: “Minnow can be ready if you didn’t invent new valves.”

The paper squid grins. You do not grin back. You pick the kind of echo you want to trust.