In Chapter XI on carbon dioxide, Geoffrey Martin discusses limestone rocks and formations. Of Mammoth Cave in Kentucky, he writes:

"One distant and dangerous abyss, called the Maelstrom, was first explored by a boy, Prentice by name. He was let down by a rope into the pitch-black depths. Down he went, where never man had been before, swinging over the giddy gulf as he went. Half way down he swung into a waterfall which nearly put out his light. Soon he was through the spray, and stood on solid rock 190 feet below. As he came up he stopped at a niche, and, leaving the rope, wandered off to explore a series of galleries and halls which opened out on to it. When he returned—oh horror!—the rope had swung off the stalactite, and was dangling beyond his reach! The plucky lad, however, did not faint; he took the wires off his lamp, twisted them together into a hook, fished back the rope, and signalled aloft to hoist away!"