Wasn’t it linear B that hasn’t been deciphered yet? Could be wrong, but one of the early Greek scripts to my knowledge (which is very amateur grade) has never been decoded.
Linear B was deciphered by Michael Ventris and John Chadwick in the early 1950s. It is apparently an archaic Greek language written in Minoan (Linear A) syllabic script. As far as I know, Linear A has not yet been deciphered.