The mussel shells the miners carried from the Green River to use as scraping tools complete a geological circle: the river that carved the cave also provided the tools that were used to harvest what the cave produced.

The Green River’s freshwater mussel populations — which were historically rich, the river being part of the biologically diverse freshwater system of the Ohio River drainage — gave the ancient miners a natural supply of the ideal mineral-scraping implement.

The Darkness As A Variable — What It Actually Meant

The darkness inside Mammoth Cave at 2.5 miles from the entrance is not a graduated darkness, not the dimness of twilight or the shadow of a clouded night sky.

It is the absence of any light whatsoever.