The moon pool is a square of silver in the floor, too calm for the amount of ocean it represents. It smells like cold metal and a beach that never got sunlight. Reed Malik crouches at the rim in a fleece the color of old lime, watching the work sled Minnow hang on its tethers like a patient insect.

“Oxygen in here is fine,” Reed says, as if you asked. “That’s not today’s trick. Today’s trick is density. Minnow wants to be a fish that forgot which way is up—near-neutral. Too heavy and we plow mud. Too light and we become a cork, and corks don’t steer.”

Colored valves line the ballast alley: blue, yellow, red, a green one with a warning collar. They look like a game. They are not a game.

Reed taps a tank. “We pump seawater in, we’re denser, we sink. We blow air in—carefully, from our 1-atm banks into the tank, displacing water—we get lighter. Air is a guest. At this depth, if you let a guest expand on the way up, the guest takes the whole house.”

Vargas on the radio: “I need a report that is not poetry.”

Reed raises an eyebrow at you. “You can help me trim the boring way, you can invent a new way with those valves, or you can sprint to sonar and leave the sled to luck.”

A strip of tape on a pipe says THINK IN KILOGRAMS. Reed wrote it after a trainee, not you, tried to be a legend.