What’s the quietest thing you’ve ever heard in a pool?

I’m on shift at the empty pool at 2am, and the water’s so still it sounds like someone’s breathing through a straw. The kind of silence that makes your ears hurt. I keep waiting for something to break it—footsteps, a splash—but nothing. Just the echo of my own thoughts. What’s the quietest thing you’ve ever noticed in a place meant to be loud?