Yuna’s mouth twitches, almost a smile. “Say the ruler out loud.”

You put it in aquanaut words. You send a ping—a short, honest sound. It hits something and comes home. Sound in seawater runs about fifteen hundred meters every second, give or take the temperature. You clock the wait. Half of that wait is the going, half is the coming. Distance is speed times time, then divide by two.

Yuna spins a little animation: a dot, a wall, a returning pulse. “She’s not wrong. The divide-by-two is the part people skip when they want to feel fast. Skip it and you think the wall is twice as far, or twice as close, and either lie can park you in rock.”

The clock in the corner still has Sprat’s missed window blinking.

“We can grid the terrace with polite pings and still make it,” Yuna says, “or we can fire one cathedral of a ping because the clock is rude. I have shouted at the ocean. The ocean shouted back in ten directions and none of them were Paz. Your call. You’re the one who wanted a ruler.”

You picture the packet they made you read on the ship: thermoclines like lids, sound bending like a stick in a glass. Nereid is a dry bubble with polyethylene and steel, not a superhero. Geometry has to do some of the work.

TIDE, uninvited: “Nominal sound speed at our depth: one thousand four hundred eighty-nine meters per second. That number will lie if you take it upstairs into the warm plume.”