One clap is a test. Two claps is data.

You stay on the safe side of the bars. You clap once, sharp.

The adit says clap. The stone hallway thinks about it. Then the hallway says clap back, thinner, later. You count: one-Mississippi, two. About two seconds. Sound is a vibration in air. Rock is a mirror for it. Priya’s rough rule rides in your pocket: sound travels about a kilometer in three seconds. This echo is a nearer wall, a pillar, a turn in the dark you will not visit.

Ellis’s headlamp wobbles. “If we rattle the lock, maybe the knockers rattle back. That’s how you know.”

“That’s how you scare yourself with your own noise,” you say, which is less movie and more true.

The air behind the grate is still and wrong-feeling in a small way, like a room that has been holding its breath. Sealed mines can hold bad air: too little oxygen, or gases that take its place. You do not need a full lecture. You need the grate to stay a grate.

A drip somewhere inside keeps a clock that is not a clock.

You put your glove on the bar, not through it. Iron bites cold through the knit. Ellis’s story wants a little person with a hammer. The hallway wants a physics problem. You give the hallway the clap and the count, and you give Ellis the respect of not laughing. Miners who listened to knocks were not foolish. They were tired people trying not to be surprised by falling rock. You can keep their habit—listen—without buying a padlock rattle.