Echo in the Flue

I hauled a smooth grey pebble up the incline of the concrete splash block at the patio corner. Right above my head, the corrugated aluminum downspout curved into a wide, open metal throat. If this channel was a viable route or an exposed flank, I needed to gauge what lay inside. Leaning over the rim, I tossed the stone into the dark bend. It struck with a sharp ping, rattling three times in hollow succession as the sound resonated up through the pipe before spitting the stone back onto the concrete. Clear conduit, no obstructions, and loud enough to serve as an early alarm.