Mapping the Lower Chasm

I leaned over the cast-iron grating, listening to the warm draft whispering through the dark vent below. A drop into an unmapped subterranean duct requires a calibrated sounding line. I unhooked my brass safety pin from my harness, sprung the catch open, and threaded the looped head through the waxed twine anchored to my pack. Clamping my right mechanical arm onto the cord, I lowered the pin through an iron square. Down into the shadows it went... until a clean, metallic clink resonated from below. Two spans deep. I marked the twine with my golden pick. The floor below the grate is dry sheet metal. Perfectly viable ground for a disciplined descent.