Strabo Geography 7.5.9

[9] Now the mouth is common to both, but the Ionian is different in that it is the name of the first part of this sea, whereas Adrias is the name of the inside part of the sea as far as the recess; at the present time, however, Adrias is also the name of the sea as a whole. According to Theopompus, the first name came from a man, ¹ a native of Issa, ² who once ruled over the region, whereas the Adrias was named after a river. ³ The distance from the country of the Liburnians to the Ceraunian Mountains is slightly more than two thousand stadia Theopompus states that the whole voyage from the recess takes six days, and that on foot the length of the Illyrian country is as much as thirty days, though in my opinion he makes the distance too great. ⁴ And he also says other things that are incredible: first, that the seas ⁵ are connected by a subterranean passage, from the fact that both Chian and Thasian pottery are found in the Naro River; secondly, that both seas are visible from a certain mountain; ⁶ and thirdly, when he puts down a certain one of the Liburnides islands as large enough to have a circuit of five hundred stadia; ⁷ and fourthly, that the Ister empties by one of its mouths into the Adrias. In Eratosthenes, also, are some false hearsay statements of this kind—“popular notions,” ⁸ as Polybius calls them when speaking of him and the other historians.

[9] The mouth is common to both; but this difference is to be observed, that the name Ionian ¹ is applied to the first part of the gulf only, and Adriatic to the interior sea up to the farthest end, but the name Adriatic is now applied to the whole sea. According to Theopompus, the name Ionian was derived from a chief (Ionius) of that country, a native of Issa; and the name Adriatic from a river, Adrias. ²

From the Liburni to the Ceraunian mountains is a distance of a little more than 2000 stadia. But Theopompus says, that it is six days’ sail from the farthest recess of the bay, but a journey of thirty days by land along the length of Ilyria. This appears to me an exaggeration, but he makes many incredible statements. Among other instances, he pretends that there is a subterraneous passage between the Adriatic and the Aegaean Seas, grounding his opinion on the discovery of Chian and Thasian pottery in the river Naron. ³ The two seas, he says, may be seen from some pretended mountain. He describes the Liburnian islands as occupying a position so extensive as to form a circle of 500 stadia. According to him, the Danube discharges itself by one of its mouths into the Adriatic. ⁴ Similar mistakes are to be found in Eratosthenes, which Polybius, when speaking of him and other writers, describes as having their origin in vulgar error. ⁵