Now if you can imagine for yourself (though I suppose a little boy cannot) the amount of lime which one of these subterranean rivers would carry away, gnawing underground centuries after centuries, day and night, summer and winter, then you will not be surprised at the enormous size of caverns which may be seen in different parts of the world--but always, I believe, in limestone rock. You would not be surprised (though you would admire them) at the caverns of Adelsberg, in Carniola (in the south of Austria, near the top of the Adriatic), which runs, I believe, for miles in length; and in the lakes of which, in darkness from its birth until its death, lives that strange beast, the Proteus a sort of long newt which never comes to perfection--I suppose for want of the genial sunlight which makes all things grow. But he is blind; and more, he keeps all his life the same feathery gills which newts have when they are babies, and which we have so often looked at through the microscope, to see the blood-globules run round and round inside. You would not wonder, either, at the Czirknitz Lake, near the same place, which at certain times of the year vanishes suddenly through chasms under water, sucking the fish down with it; and after a certain time boils suddenly up again from the depths, bringing back with it the fish, who have been swimming comfortably all the time in a subterranean lake; and bringing back, too (and, extraordinary as this story is, there is good reason to believe it true), live wild ducks who went down small and unfledged, and come back full-grown and fat, with water-weeds and small fish in their stomachs, showing they have had plenty to feed on underground. But--and this is the strangest part of the story, if true--they come up unfledged just as they went down, and are moreover blind from having been so long in darkness. After a while, however, folks say their eyes get right, their feathers grow, and they fly away like other birds. Neither would you be surprised (if you recollect that Madam How is a very old lady indeed, and that some of her work is very old likewise) at that Mammoth Cave in Kentucky, the largest cave in the known world, through which you may walk nearly ten miles on end, and in which a hundred miles of gallery have been explored already, and yet no end found to the cave. In it (the guides will tell you) there are "226 avenues, 47 domes, 8 cataracts, 23 pits, and several rivers;" and if that fact is not very interesting to you (as it certainly is not to me) I will tell you something which ought to interest you: that this cave is so immensely old that various kinds of little animals, who have settled themselves in the outer parts of it, have had time to change their shape, and to become quite blind; so that blind fathers and mothers have blind children, generation after generation.