The longest running text in the undeciphered Southwest script of Iberia was discovered recently in southern Portugal, according to an article I found today.
The script dates back to about 800-500 BC and quite a few fragments of texts have been found. Some experts believe the texts were written by pre-Roman tribes such as the Tartessians, Conii or Cynetes, others think the Celts wrote them. The script appears to be an adaptation of the Phoenician and Greek alphabets and symbols representing syllables, consonants and vowels have been identified. However as the language in question is unknown, the script has resisted all attempts to decipher it.