diving into a city that shouldn't be there

I'm swimming down a shelf of basalt and the light is doing that green-grey shiver it does at depth, only instead of a wall of rock there are windows cut into it, blue and dark, like a city that sank but kept its streetlights on. The air in my regulator tastes like copper and old pennies. I never reach the bottom, but I can feel it down there — something huge breathing, waiting for me to get closer.