He'd been dreaming, before he woke up, about the giant water-pipes, the hubbly-bubblies that the Turks smoked in Leipzig. They'd suck on the tube, and smoke from the tobacco bowl would pass down through the water and come back upwards into the tube, cooled and purified. The dream had, he guessed, been inspired by the last sound he'd heard before falling asleep, because the cave had made a similar seething and gurgling noise. As he considered it (having no other way to spend the time), he wondered whether the mine might not have acted like a giant water-pipe, and the fire like a giant Turk sucking on its tube, drawing air downwards, through a water-filled sump, from the outside, so that it bubbled up into this tunnel.