The sonar shack smells like warm plastic and ginger tea. Dr. Yuna Sato is already in the chair, hair in a knot that has survived worse watches. Her hands hover over the ping board without touching it yet.

“Aquanaut,” she says, not unkind. “Talk to me like you mean it. Sprat is a small 1-atm can with a person in it. Paz has an acoustic phone if the water is willing. Lights will not find them. Lights are a polite cough. Sound is a shout that comes back.”

On the main screen, a fan of pale wedges paints the terrace and the drop beyond. A smaller screen shows a wiggly line: sound speed versus depth. It is not a straight line. It never is.

Reed’s voice leaks from a speaker, arguing with a tank somewhere. Yuna ignores it. “Three honest options. We ping on a grid and let travel time be a ruler. We throw Minnow into the dark with lamps because lamps feel like doing something. Or we ask TIDE for the water’s actual speed profile before we trust any echo. You don’t have to be loud. You have to be right.”

You hook a socked foot under the seat so you do not drift—no: this is one-g, the habitat sits on skids, your body is ordinary and heavy. That itself feels like a trick after the pool work.

“Nine hundred meters is real,” Yuna adds. “Pressure does not care that you are eleven-going-on-professional. Pressure cares about depth. Sound cares about temperature. Be the person who cares about the same things.”

A sticky note on the console says DO NOT BE CLEVER in Vargas’s handwriting. Someone, not you, once cranked ping power to “wake the fish.”