Around 1,000 Greek words cannot be traced to any Indo-European language, including quintessential terms like λαβύρινθος (labýrinthos) 'labyrinth', ἔλαια (elaía) 'olive tree', Ἀχιλλεύς (Akhilleús) 'Achilles', Ὀδυσσεύς (Odysseús) 'Odysseus', Κόρινθος (Kórinthos) 'Corinth', Ἀθῆναι (Athênai) 'Athens', Ἀθήνη (Athḗnē) 'Athena', and Ἄρης (Árēs) 'Ares'. Despite centuries of effort, linguists have not linked these words to Proto-Indo-European or neighboring languages. The article explores the hypothesis that these lexical remnants derive from Paleo-European (Old European) languages spoken by Neolithic farming societies before Indo-European migrations.

As early Hellenic speakers migrated from the Pontic–Caspian Steppe into the eastern Mediterranean, they encountered unfamiliar flora, fauna, and terrain—such as olives, figs, cypresses, and the sea—and adopted or adapted words from the substrate languages of the region. This substrate influence left a lasting mark on Ancient Greek, preserving traces of a lost linguistic layer beneath the Indo-European superstrate. The mystery language, possibly related to Paleo-European tongues like Etruscan or Iberian, remains unidentified, though Basque is the sole surviving descendant of this ancient linguistic stratum.