The History Of Herodotus Volume 2 of 2
Page: 117129. Now it is said that Thessaly was in old time a lake, being enclosed on all sides by very lofty mountains: for the parts of it which lie towards the East are shut in by the ranges of Pelion and Ossa, which join one another in their lower slopes, the parts towards the North Wind by Olympos, those towards the West by Pindos and those towards the mid-day and the South Wind by Othrys; and the region in the midst, between these mountains which have been named, is Thessaly, forming as it were a hollow. Whereas then many rivers flow into it and among them these five of most note, namely Peneios, Apidanos, Onochonos, Enipeus and Pamisos, these, which collect their waters from the mountains that enclose Thessaly round, and flow into this plain, with names separate each one, having their outflow into the sea by one channel and that a narrow one, first mingling their waters all together in one and the same stream; and so soon as they are mingled together, from that point onwards the Peneios prevails with its name over the rest and causes the others to lose their separate names. And it is said that in ancient time, there not being yet this channel and outflow between the mountains, these rivers, and besides these rivers the lake Boibeïs also, had no names as they have now, but by their waters they made Thessaly to be all sea. The Thessalians themselves say that Poseidon made the channel through which the Peneios flows; and reasonably they report it thus, because whosoever believes that it is Poseidon who shakes the Earth and that the partings asunder produced by earthquake are the work of this god, would say, if he saw this, that it was made by Poseidon; for the parting asunder of the mountains is the work of an earthquake, as is evident to me.
130. So the guides, when Xerxes asked whether there was any other possible outlet to the sea for the Peneios, said with exact knowledge of the truth: "O king, for this river there is no other outgoing which extends to the sea, but this alone; for all Thessaly is circled about with mountains as with a crown." To this Xerxes is said to have replied: "The Thessalians then are prudent men. This it appears was that which they desired to guard against in good time 111 when they changed their counsel, 112 reflecting on this especially besides other things, namely that they had a country which, it appears, is easy to conquer and may quickly be taken: for it would have been necessary only to let the river flow over their land by making an embankment to keep it from going through the narrow channel and so diverting the course by which now it flows, in order to put all Thessaly under water except the mountains." This he said in reference to the sons of Aleuas, because they, being Thessalians, were the first of the Hellenes who gave themselves over to the king; for Xerxes thought that they offered him friendship on behalf of their whole nation. Having said thus and having looked at the place, he sailed back to Therma.