You strap into the observer seat—habit from the pool—and tap the call. “TIDE. Storm on the surface, not here. I need sound speed versus depth, and what our pings actually do. Short.”

“Regional surface storm is irrelevant to acoustic travel times at nine hundred meters,” TIDE says. “A warm, salty plume over the vents is not. Sound speed increases with temperature and, less so, with salinity. A thermocline can refract rays. A strong ping may return from a bent path and paint a ghost at a false range.”

A diagram blooms: Nereid as a square, the terrace as a step, rays like bent straws.

“Range estimate equals one-half sound speed times two-way travel time. Use the speed of the water the sound actually swam in, not the speed you memorized in a classroom with sticky chairs.”

Yuna translates, fingers flying. “Classroom number is a handshake. Profile is a promise. Moon trip—no. Vent trip in a plume is like reading a street sign in a funhouse mirror.”

Vargas: “Data is a tool. Still your move.”

You ask one more question, because good questions are free. “TIDE, is ‘just go look with lamps’ ever the long-range answer?”

“For bolt-turning at two meters, yes,” TIDE says. “For a lost vehicle beyond lamp range, no. Empty water is not a window. Empty water is empty, and then it is a wall you did not see.”

Yuna taps the paper squid as if it were a good-luck charm, then stops, because luck is not a procedure.