The way to Petries is somehow hidden for normal tourists. You drive from Athen to Oropos, then you get the ferry to Eretria and then drive to Aliveri, and finally you drive towards the Agais sea in east direction. The GPS is your best friend.
Μηλακι Αλιβεριου (μπροστα) - Πετριες (στο βαθος)
