From the Wikipedia page 
The Youra Potsherds (or Gioura Potsherds) are ceramic fragments dated to 6000 BC - 5500 BC discovered during systematic explorations in the "Cyclops Cave" at the uninhabited islet of Youra (20 miles from Alonissos) in the northern Sporades, an Aegean archipelago off the coast of Thessaly in Greece. These fragments were discovered in 1992 by Adamantios Sampson, Inspector of Antiquities, during a project whose more general purpose was to clarify the prehistoric occupation sequence in the area, with an emphasis on the pre-pottery sequences from the Late Pleistocene, especially two thick areas of deposits: Late Aegean Neolithic Ib and Early Neolithic II.
Some of the incisions on the potsherds bear some apparent resemblance to letter shapes. Adamantios Sampson notes that the "vase bear[s] incised unidentifiable symbols. It is possible that it echoes evidence on an Aegean Neolithic 'script' or 'proto-script', a very fashionable subject of discussion in Greece, after similar finds in Kastoria lake, East [sic] Macedonia."