The second and next largest department is that of the "chambers." These are places in the general course of the former river where the roof fell in before the withdrawal of the waters, opening great spaces upward; the fallen mass forming a sort of island in the centre of the stream, and crowding the waters on either side of it against the walls of the cave, so that they were worn out to twice the average width, and finally itself disappearing under the combined action of the current and the solvent properties of the water.
The air in all these tunnels and chambers is remarkably dry and pure. Wood seems never to decay here; as is instanced in the wooden pipes and vats of the saltpetre-makers, upon which the lapse of a half-century has not had any visible effect.
The general width of the tunnels or avenues is about forty or fifty feet, and the average height about thirty feet; but this uniformity is broken every few hundred yards by chambers, varying in width from eighty to two hundred feet, and in height from seventy to two hundred and fifty feet. The floor is formed in some places of sand, but generally of indurated mud, so hard that it is impossible to make any indentation in it with the heel of the boot, and remarkably even and smooth, so that almost anywhere one can walk with as much ease as on city sidewalks. The walls also are clean and smooth, as in the arched crypts of some mighty cathedral. A cross section of almost any one of these tunnels would show an elliptic outline, the vertical diameter being the shortest, and the bottom being filled with indurated mud or sand to a sufficient depth to make a level floor.
The third division or class of openings is that of the "domes" and "pits." These were formed by the same kind of agency as the tunnels and chambers, namely, by the action of water holding carbonic acid in solution; but acting in a different manner, and at a period long after the subterranean river had ceased to flow through its tunnels.
The solvent acid of the water must be acquired in percolating through the several hundreds of feet of superincumbent earth and sandstone, as there is but one of these domes, "Sandstone Dome," that extends upward to the sandstone. The solvent water then, after finding its way into the vertical crevices of the limestone, gradually rounded them out like wells, until the pieces which occasionally fell from the top formed a sort of floor. Through the interstices of this floor, the dissolved substance of the rock is carried into some deeper and yet undiscovered cavities beneath. The floor itself gradually sinks, the domes grow higher, and the walls recede as long as the water continues to drip into them.
It is not unreasonable to suppose that the substratum of limestone in all the country for many miles in the vicinity is perforated with these tremendous shafts; as those which are seen in the cave are only such as happened to be in the course of the tunnels composing the Mammoth Cave. It is not improbable that there are many others on every side, close to the line of the tunnels, but not yet connected with them.
In any one of the above-mentioned departments, the description of one place answers for most others, except in dimensions. The following are a few out of many hundreds of measurements taken in as many different places:—
The "Rotunda," the first chamber from the entrance of the cave, is about one hundred feet high, and one hundred and seventy-five in diameter.
The "Methodist Church," eighty feet in diameter, and forty in height.
"Wright's Rotunda" is four hundred feet in its shortest diameter, being nearly circular; the roof seems perfectly level, and is about forty-five feet high.