Dead languages

Particularly over the past century or so, most of the known written human languages from every point in history have successfully been decoded. But to do this has essentially always required finding a case where there is explicit overlap with a known language—say a Rosetta stone with the same text in multiple languages, or at least words or proper names that are transliterated. As in cryptanalysis, it is sometimes remarkable how small an amount of text is needed to find a decoding scheme. But usually what is done relies critically on the slowness with which human languages change, and the comparatively limited number of different basic ways in which they work.