Herodotus 7.22: καὶ τοῦτο μέν, ὡς προσπταισάντων τῶν πρώτων περιπλεόντων περὶ τὸν Ἄθων προετοιμάζετο ἐκ τριῶν ἐτέων κου μάλιστα τὰ ἐς τὸν Ἄθων.

"Since those who had earlier attempted to sail around Athos had suffered shipwreck, for about three years preparations had been underway there."

Those others being referred to here are Mardonius' expedition in 492. Herodotus tells us he lost 300 ships and 20,000 men sailing around said peninsula. Three years seems like a long time, but 300 ships and 20,000 men was huge.

Herodotus 6.44: ἐκ μὲν δὴ Θάσου διαβαλόντες πέρην ὑπὸ τὴν ἤπειρον ἐκομίζοντο μέχρι Ἀκάνθου, ἐκ δὲ Ἀκάνθου ὁρμώμενοι τὸν Ἄθων περιέβαλλον. ἐπιπεσὼν δέ σφι περιπλέουσι βορέης ἄνεμος μέγας τε καὶ ἄπορος κάρτα τρηχέως περιέσπε, πλήθεϊ πολλὰς τῶν νεῶν ἐκβάλλων πρὸς τὸν Ἄθων. λέγεται γὰρ τριηκοσίας μὲν τῶν νεῶν τὰς διαφθαρείσας εἶναι, ὑπὲρ δὲ δύο μυριάδας ἀνθρώπων. ὥστε γὰρ θηριωδεστάτης ἐούσης τῆς θαλάσσης ταύτης τῆς περὶ τὸν Ἄθων, οἳ μὲν ὑπὸ τῶν θηρίων διεφθείροντο ἁρπαζόμενοι, οἳ δὲ πρὸς τὰς πέτρας ἀρασσόμενοι: οἳ δὲ αὐτῶν νέειν οὐκ ἐπιστέατο καὶ κατὰ τοῦτο διεφθείροντο, οἳ δὲ ῥίγεϊ.

"Crossing over from Thasos they travelled near the land as far as Acanthus, and putting out from there they tried to round Athos. But a great and irresistible north wind fell upon them as they sailed past and dealt very roughly with them, driving many of their ships upon Athos. It is said that about three hundred ships were lost, and more than twenty thousand men. Since the coasts of Athos abound in wild beasts, some men were carried off by beasts and so perished; others were dashed against the rocks; those who could not swim perished because of that, and still others by the cold."