ON THE ORIGIN AND CAUSES OF SUBTERRANEAN THINGS, BOOK I

...we understand that of the waters which are under the earth, some are collected from rains, others arise from vapor, and others flow from the sea or rivers. Since these things are so, fountain water—that is, water erupting from the earth of its own accord—frequently arises; not rarely is it collected from rains; very rarely does it flow from the sea or nearby rivers. But well water, that is, water which does not flow from the earth of its own accord but is drawn from it, frequently flows from the sea or deep rivers; not rarely is it collected from rains; it very rarely has an [independent] origin. Fountain water, because it frequently arises, is not lacking in the hottest regions; because it is not rarely collected from rains, regions situated under the North or South have many more and larger springs than the hottest ones, for their earth is not only moistened by water that flows from the sea or rivers, but also by rains; whence, having been heated, it generates more exhalations.

But because fountain water very rarely flows from the sea or nearby rivers, few salt springs are found in maritime places, or sweet ones arising in the flat plains, for it is necessary that the sea or river be higher than the spring's mouth. Such salt springs, however, generally grow when the sea waters have approached by tide, and decrease when they have retreated. But sweet springs, as soon as rivers have dried up in extreme drought, are diminished or lack waters. Nor are those springs perennial whose entire water is either collected from rains or arises from vapors which the earth, damp with rains, emits when it has heated up. But as for well water, because it frequently flows from the sea or from rivers which irrigate the interior earth, the hottest regions do not lack it, provided the inhabitants dig the wells deeper. Because the same is not rarely collected from rains, those who inhabit regions abundant in rains have no need to dig wells deeper so that water may emerge from the sides. Finally, because it very rarely has an origin, the shallow wells of the hottest regions also, for this reason, generally lack waters. Nevertheless, sometimes when there are caverns in the earth, they pour water generated by origin through veins or veinlets into the wells. For when wells are dug, all nearby water, whether it has flowed from the sea or rivers, or has arisen from vapors, or has been collected from rains, descends into the deeper place.

But enough of these also: now I come to the effects of waters. They themselves excavate those things which are inside the earth, just as those which are outside, so that they make not only channels but also huge caverns. This should seem surprising to no one, since geographers tell us not only of springs, but also of certain greatest rivers which penetrate under the earths and enter their caves, and are hidden in them; and thus enclosed and hidden, they lie, as it were, in the occult over a long interval of places, and again suddenly leap forth and return from the darkness into the light. For example: the Nile in Ethiopia, and the Tigris in Greater Armenia: so that not...