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Some, who, like Thales, attribute earthquake to the effects of water, give a different explanation of its operation. There are, they say, many kinds of waters running over the whole earth. In one place there are constant rivers whose size renders them fit for navigation, even without the aid of rains. There is the Nile, rolling down its huge volume all summer long: here are the Danube and the Rhine separating with their streams the peaceful from the hostile, the former checking attacks from the Sarmatians and forming the boundary between Europe and Asia, the latter keeping back the Germans, a nation ever keen for war. Then there are lakes of very wide extent, great pools surrounded by tribes mutually ignorant of each other, marshes that no boat can struggle through, that cannot be passed even by the people that dwell on their borders. Add, then, the multitude of fountains, and of river sources that belch out of their recesses full-grown streams. Besides, there are many rushing torrents that gather only for a time, whose force is as shortlived as it is sudden. Now there are waters, in all this variety of form and character, within as well as above the earth. Away there below some are borne along in vast bulk, and tumble their whole volume down the steep: others more sluggish are dammed back in shallows, and flow with gentle, quiet stream. And can any one deny that within those vast underground hollows waters are formed, and lie sluggish and inactive in many places? It needs no long proof to show that there must be many waters in the place where all waters are. The earth would not be able to produce so many rivers unless it poured them from a copious reserve.
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This being so, sometimes below the earth a river must become swollen, and leaving its banks assail with violence all obstacles that meet it. So there will be a movement of some point on which the river has made an onset, and which it will keep lashing until its waters fall. Or it may happen that the constant wear of a stream may eat away some quarter, dragging down thereby some mass above, by whose fall, in turn, the surface which rested on it is shaken. Now surely a man trusts too much to the sight of the eyes and cannot launch out his imagination beyond, if he does not believe that the depths of earth contain a vast sea with winding shores. I see nothing to prevent or oppose the existence of a beach down there in the obscurity, or a sea finding its way through the hidden entrances to its appointed place. There, too, it occupies as much space as here, perhaps more, since the regions up on earth have had to be shared with so many living creatures; but the hidden regions being desert without inhabitant give freer scope to the waves of the nether ocean. And who is there to hinder the sea from swelling there and being tossed by all the winds that every interstice of the earth, and every species of atmosphere can create? So, then, when a storm greater than ordinary has arisen, it may beat upon some one side of the earth with too great vehemence and move it. For on the surface likewise, many places which had been far from the sea have felt the violence of its sudden approach: villas almost out of sight of it have been invaded by the waves which used only to be heard in the distance. The nether sea, too, can approach and retire; neither of which movements can take place without shock to the earth that stands above it.