But to return to my subject. Here is a proof I have to give you 1 that in the underground recesses are concealed great quantities of water which abound in filthy fish. Any time that the water bursts out, it brings in its train a huge crowd of creatures foul to sight, disgusting and noxious to taste. At any rate, once, near the city of Hydissus in Caria, a flood of underground water threw up to the light of day a number of strange fishes, and all who ate them died. And no wonder. Their bodies were full of oil from their long inactivity; 2 they had been fattened in the darkness without exercise, and deprived of that light whence health is derived. A further proof that fish may be produced in those depths of earth is afforded by the breeding of eels in shady places; they also are a heavy diet through their want of exercise, especially if a considerable depth of mud has hidden them quite out of sight. So then the earth contains not only veins of water 3 by the union of which rivers may be formed, but also streams of very great size. In some cases their channel is concealed throughout, until they are swallowed up in some cavern; others of them well up in the bottom of some lake. Everybody knows that some marshes have no bottom. What is the point of my argument? It shows plainly that mighty rivers have here unending supplies whose limits are incalculable, just as is the duration of rivers and fountains themselves. XX For the variety of taste in water there are four causes. The first is 1 the kind of soil through which it flows. The second also depends on the soil when the water arises from transmutation of it. The third is from air which has been transformed into water. The fourth comes from some taint which water often contracts when injuriously affected by foreign bodies. These causes impart to water, first, variety of taste, then 2 medicinal power, its heavy pestilential smell, its lightness and heaviness, its heat and its excessive astringency. It is affected by its passage through ground full of sulphur, or nitre (= saltpetre), or bitumen. If the water is tainted in this way, the drinking of it endangers life. This is the explanation of a passage in Ovid: The Ciconians have a river a draught of whose waters turns into stone. The bowels; which mantles in marble all that it touches.