River Hall, to which we now return, might be said to extend for miles, were we to include all the known branches of subterranean waters. So unlike is it to the Main Cave that we might almost be said to have entered another cavern—which would really be true. What is called in a general way Mammoth Cave is a congeries of different caves, whose walls and floors were first thinned and then broken through by the agency of water, until was formed the immense and greatly diversified labyrinth whose mazes we are exploring. Here is the gathering-bed of hundreds of sink-holes opening from the surface. The exit is in deep, bubbling pools along Green River, of which the Upper and Lower Big Springs are examples. And when Green River is flooded by freshets its waters back through such secret channels and also flood River Hall by a body of water from thirty to one hundred feet in depth and fully two miles long, with capricious currents and perilous whirlpools. Navigation at such times is forbidden; but at low water it is entirely safe, under the care of our skillful guides. Sullen waters reposing at the foot of a cliff sixty feet high are called the Dead Sea, though not bitter but sweet, as those may find who venture down to the margin. An iron railing guards the way as we descend to a lower terrace. Presently, on the right, we see a cascade, that falls into a funnel-shaped hollow and vanishes. Near by, in 1881, the writer found a natural mushroom bed, that suggested the idea of a mushroom farm, but with meager results because located in Audubon Avenue, where irrigation is impracticable. The black waters of the River Styx wind between steep walls for some four hundred feet, and with an average breadth of forty feet. Formerly it was passed over by boat, but now by a natural bridge protected by a guard-rail. Lake Lethe is next in order, along whose border we go cautiously, in hope of seeing specimens of the famous eyeless fish (_Amblyopsis speleus_) that abound in these waters. They seldom exceed three or four inches in length, are colorless, have cartilage instead of bones, are viviparous, and are so sensitive to approach that they dart away if a grain of sand falls on the water. The blind white crawfish (_Cambarus pellucidus_) is often seen. These creatures were first described by Dr. Davidson, two years previous to their being mentioned by DeKay, who was credited by Agassiz as their discoverer. [Illustration: Banquet Hall in Mammoth Cave]