Some of these plains are marshy, since rivers spread out over them, though other rivers fall into them and later find a way out; other plains are dried up, and on account of their fertility are tilled in all kinds of ways. But since the depths of the earth are full of caverns and holes,1
it has often happened that violent earthquakes have blocked up some of the passages, and also opened up others, some up to the surface of the earth and others through underground channels. The result for the waters, therefore, is that some of the streams flow through underground channels, whereas others flow on the surface of the earth, thus forming lakes and rivers. And when the channels in the depths of the earth are stopped up, it comes to pass that the lakes expand as far as the inhabited places, so that they swallow up both cities and districts, and that when the same channels, or others, are opened up, these cities and districts are uncovered; and that the same regions at one time are traversed in boats and at another on foot, and the same cities at one time are situated on the lake2
and at another far away from it.