At the beginning of the 20th century, archaeologist Sir Arthur Evans began excavating Knossos, an ancient city located on the island of Crete. In doing so he uncovered a great many clay tablets inscribed with a previously unknown script. Some were older and were named Linear A. The bulk were of more recent vintage, however, and were dubbed Linear B. Evans spent the next several decades trying to decipher both to no avail.
Part of the difficulty stemmed from Evans himself -- he had strong opinions about the nature of Cretan civilization, and was convinced that Linear B was used to write a hypothetical language he named "Minoan." A powerful political force in the academic world, Evans succeeded in cutting off any investigation into the possibility that the language being written on the tablets was Greek, despite some hints that this was the case.
Shortly after Evans died, it was noted that certain words in Linear B inscriptions had changing word endings -- perhaps declensions in the manner of Latin or Greek. Using this clue, Michael Ventris constructed a series of grids associating the symbols on the tablets with consonants and vowels. While which consonants and vowels they were remained mysterious, Ventris learned enough about the structure of the underlying language to begin guessing.
Some Linear B tablets had been discovered on the Greek mainland, and there was reason to believe that some of the chains of symbols he had encountered on the Cretan tablets were names. Noting that certain names only appeared in the Cretan texts, he made the inspired guess that those names applied to cities on the island. This proved to be correct. Armed with the symbols he could decipher from this, Ventris soon unlocked much text, and determined that the underlying language of Linear B was in fact Greek. This overturned Evans' theories of Minoan history, by establishing that Cretan civilization, at least in the later periods associated with the Linear B tablets, had been been part of Mycenean Greece.
A few years after deciphering Linear B in 1951-1953, Ventris died in a car crash aged 34.