Although the passes of the Alps had been used for trans-European commerce since prehistoric times, the early Greeks had no knowledge of these mountains, though a vague notion of them may lurk in their speculations about the *Hercynian Forest and the *Rhipaei montes; in Herodotus (4. 49) ‘Alpis’ is a tributary of the Danube. By the 4th cent. Greek travellers in north Italy and Provence brought information about a ‘pillar’ or ‘buttress’ of the north (Ephorus, in Scymn. 188); but *Apollonius (1) Rhodius (4. 627 f.) could still believe that the Rhône (Rhodanus) and Po (*Padus) were interconnected. The Roman conquest of *Cisalpine Gaul and Hannibal's invasion of Italy (Polyb. 3. 50–6; Livy 21. 32–7; the pass remains unidentified) brought more detailed knowledge, and *Polybius (1) gave a good description of the western Alps, though he thought that they extended uniformly in a west–east direction. The campaigns of *Caesar in Gaul, and of *Tiberius in Switzerland and Austria, opened up the Alps thoroughly, and in the first two centuries ce at least five paved roads (Little and Great St Bernard, Splügen, Maloja, and Brenner passes) were built across them.