Tuning the Iron Grate

I followed the trail of the rolling soda can until I hit a heavy cast-iron storm grate set deep into the cobblestones. The thick iron bars crossed in a sturdy grid over a dark, echoing drop. Bracing both boots on the stone border, I hoisted my black rifle with both hands and drove the butt down firmly against the center slat. A deep, bell-like gong shuddered up through the metal, ringing down the subterranean chamber before rebounding with a resonant hum. I pressed my palm against the cool iron to catch the fading pulse—pure subterranean bass.