Some writers think there is an exact parallelism between the external and the internal distribution of water in the earth. On the outer surface are huge marshes, great navigable lakes, and seas covering immense tracts of earth and pouring over its hollows. So in the interior of the earth there is abundant store of fresh water, which overflows great spaces no less than the Ocean and its gulfs above ground; in fact, still more extensively, as the depth of the earth extends farther down than that of the sea. From that supply in the deeps, therefore, those rivers of which we have spoken issue. And why should one be surprised that the earth is not sensible of their withdrawal since the sea is not sensible of their addition? IX Some approve the following explanation: The earth contains, they 1 assert, many hollow recesses and a great quantity of air. This air, under pressure of the gross darkness, of necessity freezes. Then remaining sluggish and unmoved it ceases to circulate and turns into water. Just as on earth a change in the density of the atmosphere produces rain, so beneath the earth the change of density starts a river or a stream. In the former case the air above our heads cannot long remain sluggish and heavy; for sometimes it is rarefied by the sun’s heat, sometimes expanded by the wind’s force. There are, therefore, long intervals between falls of rain. But underground 2 the forces, whatever they are, that turn air into water, are constant--perpetual darkness, everlasting cold, inert density; they can, therefore, supply without a break the sources of fountain or flood. We Stoics are satisfied that the earth is interchangeable in its elements. So all this air that she has exhaled in her interior, since it is not taken up by the free atmosphere, condenses and is forthwith converted into moisture. X There you have the first cause of the origin of underground water. 1 You may add the more general principle that all elements arise from all: air comes from water, water from air; fire from air, air from fire. So why should not earth be formed from water, and conversely water from earth? If the earth is capable of transmutation into other elements, water must be one of them, in fact, the most suitable of them. The two things are cognate; both are heavy, condensed, both driven by nature down to the very confines of the universe. Earth is formed from water; why not water from earth in like manner?