Five minutes' farther walk brought us to a wooden footbridge,--a narrow, shaky contrivance, with a treacherous footing and a slender hand-rail. Here the bottom of the cave seemed to have dropped out, and the roof to have gone in search of it; and but for the dim glimpse of the rock on the other side one might have suspected that this bridge would launch him into that ungeographical locality called, in the old Norse mythology, "Ginnunga Gap,"--a place where there was neither side, edge, nor bottom to anything. The vault overhead is called "Minerva's Dome"; the gulf below is called the "Side-Saddle Pit," though I failed to discover any degree of appropriateness in the odd name. Standing in the middle of the bridge, my guide flung one of his Bengal lights far upward, in the midst of the slow-falling drops that had already carved out this tremendous well and were still making it larger. The light turned them for an instant into a shower of diamonds; then down it fell, down, down! As in its descent it passed the bridge on which we stood, the shadows of our two figures rushed up the opposite wall, like a pair of demons scared out of their abode by the hissing flame; and Nick, the guide, as he leaned over, looking downward after it,--every one of the innumerable wrinkles in his black face made more distinct, with his white beard and mustache, and the whites of his eyes seeming to glow in the blue elfish light,--was a caricature, half grotesque, almost terrible, of Satan himself. Minerva's Dome and Side-Saddle Pit, both being one place and formed by the same dripping water, correspond to Gorin's Dome and the pit beneath it; that part which has been hollowed out above the roof of the cave being called the dome, and the part below the floor of the cave the pit. The only difference between the two is that in the case of Gorin's Dome the dripping waters have bored their huge shaft on one side of the track of the cave, only just piercing the wall of it in one spot, to make the window through which it is viewed; while in the case of the Side-Saddle Pit the vertical shaft cuts directly across the track of the cave, or, to speak more correctly, across the tunnel which was once the bed of a subterranean river, but which is now a broad, smooth, dry path. The topography of this underground realm may be divided into three departments, as follows:--