As the debate over the Indus script – on if it is a language or not and if so, is it Dravidian or Indo-European – continues, New Scientist has an article listing seven other scripts that cannot be read.
Proto-Elamite is the world’s oldest undeciphered script – assuming that it really is a fully developed writing system, which is by no means certain. It was used for perhaps 150 years from around 3050 BC in Elam, the biblical name for an area that corresponds roughly to today’s oilfields of western Iran. It is almost as old as the oldest writing of all, the earliest cuneiform from Mesopotamia. Little is known about the people who wrote the script.[Decoding antiquity: Eight scripts that still can’t be read – life – 27 May 2009 – New Scientist]
I happened to read a paper on the science of deciphering unknown scripts in ancient India. The paper was based on a rather sombre poem (11th century I believe) about the scene after the destruction of the universe by Shiva. The whole universe is strewn with the skulls and bones of all the people, animals, gods, etc with names of the deceased written in an extinct language written on the skull with blood. And the Shiva-gaNAs are having fun deciphering the extinct language, with one of them figuring out to his delight that the skull he is holding is that of Indra’s.