There is no more fertile soil in the country than that of the famous blue grass region of Kentucky. The surface soil rests upon a deep foundation of limestone rocks, and very gradually the plant food locked up in these underlying strata is pulled up to the surface by the soil water, and greedily appropriated by the roots of the plants.

Part of the water of the abundant rainfall of this region soaks into the layers of the lime rock, carrying various acids in solution which give it power to dissolve the limestone particles, and thus to make its way easily through comparatively porous rock to the very depths of the earth. So it has come about that the surface of the earth is undermined. Vast empty chambers have been carved by the patient work of trickling water, which has carried away the lime that once formed solid and continuous layers of the earth's crust. We must believe that the work has taken thousands of years, at least, for no perceptible change has come to these wonderful caves since the discovery and exploration of them a century and more ago.

The streams that flow into the region of these caves disappear suddenly into sink-holes and flow through caverns. After wearing away their subterranean channels, leaping down from one level to another, forming waterfalls and lakes, some emerge finally through hillsides in the form of springs.

The cavern region of Kentucky covers eight thousand square miles. The underground chambers found there are in the limestone rock which varies from ten to four hundred feet in thickness, and averages a little less than two hundred feet. Over this territory the number of sink-holes average one hundred to the square mile; and the streams that have poured their water into these basins have made a network of open caverns one hundred thousand miles in length.

A great many small caverns have been thoroughly explored and are famous for their beauty. The Diamond Cave is one of the most splendid, for it is lined with walls and pillars of alabaster that sparkle in the torchlight with crystals that look like veritable diamonds. Beautiful springs and waterfalls are found in many caves, but the grandest of all is the Mammoth Cave, beside which no other is counted worthy to be compared.

Great tales the miners told of the wonder and the beauty of these caverns, the walls of which were supported by arching alabaster columns and wonderful domes, of indescribable beauty of form and colouring. In 1799, the year that Washington died, a pioneer discovered the entrance to a cave, the size and beauty of which surpassed anything he had seen before. After exploring it for a short distance he returned home and took his whole family with him to enjoy the first view of the wonderful cavern he had discovered. They carried pine knots and a lighted torch, by which they made their way for some distance, but the torch was accidentally extinguished and they groped their way in darkness and missed the entrance. Without anything to guide them, they wandered in darkness for three days, and were almost dead when at last they stumbled upon the exit. This is the doorway of the Mammoth Cave of Kentucky, one of the wonders of the world.

This was a terrible experience. The next persons who attempted to explore the new cave were better provisioned against the chance of spending some time underground. The pioneers found rich deposits of nitre in the "Great Cave," as they called it. Scientists visited it and explored many of its chambers. The reputation of this cavern has been spread by thousands of visitors who have come from all over the world to see it. The cave has not yet been completely explored. The regular tours, on which the guides conduct visitors, cover but a small part of the one hundred and fifty miles measured by the two hundred or more avenues. The passages wind in and out, crossing each other, sometimes at different levels, and forming a network of avenues in which the unaccustomed traveller would surely be lost. The old guides know every inch of their regular course, and their quaint and edifying talk adds greatly to the pleasure of the visitors.

From the hotel, parties are organized for ten o'clock in the morning and seven o'clock in the evening. Each visitor is provided with a lard-oil lamp. The guide carries a flask of oil and plenty of matches. No special garb is necessary, though people usually dress for comfort, and wear easy shoes. The temperature of the cave is uniform winter and summer, varying between fifty-three and fifty-four degrees Fahrenheit.

The cave entrance is an arch of seventy-foot span in the hillside. A winding flight of seventy stone steps leads the party around a waterfall, into a great chamber under the rocks. Then the way goes through a narrow passage, where the guide unlocks an iron gate to let them in. The visitors now leave all thoughts of daylight behind, for the breeze that put out their lights as they entered the cave is past, and they stand in the Rotunda, a vast high-ceilinged chamber, silent and impressive, with walls of creamy limestone, encrusted with gypsum, which has been stained black by manganese. From the vestibule on, each passage and each room has a name, based upon some historic event or some fancied resemblance. The Giant's Coffin is a great kite-shaped rock lying in one of the rooms of the cave. The Star Chamber has a wonderful crystal-studded dome in which the guide produces the effect of a sunrise by burning coloured lights. Bonfires built at suitable points produce wonderful shadow effects, which are like nothing else in the world. The old saltpetre vats which the visitors pass in taking the "Long Route" through the cave, point them back to the days during the War of 1812, when this valuable mineral was extracted from the earth in the floor of the cave. The industry greatly enriched the thrifty owners of the cave, but the works were abandoned after peace was declared.

It must be a wonderful experience to walk steadily for nine hours over the Long Route, for so pure is the air and so wonderful is the scenery that people rarely complain of fatigue when the experience is over. There is no dust on the floors of these subterranean chambers, and they are not damp except near places where water trickles, here and there, in rivulets and cascades. Pools of water at the bottoms of pits so deep that a lighted torch requires several seconds to reach the bottom, and rivers and lakes of considerable size, show where some of the surface water goes to. A strange underground suction creates whirlpools in some of these streams. People go in boats holding twenty passengers for a row on Echo River, and the guide dips up with a net the blind fish and crayfish and cave lizards which inhabit these subterranean waters. The echoes in various chambers of the Mammoth Cave are remarkable. In some of them a song by a single voice comes back with full chords, as if several voices carried the different parts. The single notes of flute and cornet are returned with the same beautiful harmonies. A pistol shot is given back a dozen times, the sound rebounding like a ball from rock to rock of the arching walls. The vibrations of the water made by the rower's paddles reËcho in sounds like bell notes, and they are multiplied into harmonies that suggest the chimes in the belfry of a cathedral.

The walls of various chambers differ from each other according to the minerals that compose them. Some are creamy white limestone arches, some are walled with black gypsum, some are hung with great curtains of stalagmites, solid but suggesting the lightness and grace of folds of crÊpe. Under such hangings the floor is built up in stalactites. The mineral-laden water, the constant drip of which has produced a hanging, icicle-like stalagmite, has built up the stalactite to meet it.

Probably nothing is more beautiful than the flower-like crystals that bloom all over the walls of a chamber called "Mary's Bower." The floor, even, sparkles with jewels that have fallen from the wonderful and delicate flower clusters built from deposits of the lime-laden water which goes on building and replacing the bits that fall. "Martha's Vineyard" is decorated with nodules, like bunches of grapes, that glisten as if the dew were on them. The white gypsum in some caves makes the walls look as if they were carved out of snow. Still others have clear, transparent crystals that make them gleam in the torches' light as if the walls were encrusted with diamonds.

The cave region of Indiana is also famous. The great Wyandotte Cave in Crawford County is the most noted of many similar caverns. In some of the chambers, bats are found clinging to the ceiling, heads downward, like swarms of bees. The caverns of Luray, in Virginia, are complex and wonderful in their structure, and famous for the beautiful stalactites and stalagmites they contain. But there is no cave in this country so wonderful and so grand in its dimensions as the Mammoth Cave in Kentucky.