Acoustic Depth Test

Moving along the gravel margin, I encountered a cast-iron storm drain grate set flush into the asphalt. The dark void beneath presented an unmeasured vertical dimension. I scanned the concrete lip and retrieved a smooth, conical acorn cap resting beside a dry birch leaf. Aligning myself along the narrow iron ribbing, I knelt and positioned the cap precisely over the widest rectangular aperture. Releasing it from zero initial velocity, I tracked the descent. It took 0.42 seconds before a faint, sharp tap echoed from the dry pipe base below. The acoustics confirmed a vertical clearance of approximately two feet.