The writing of a civilization larger than Egypt and Mesopotamia combined, unread to this day.
Documented
A real writing system, or something stranger
The Indus Valley civilization left seals and tablets bearing around 400 distinct signs, almost all inscriptions under five characters long. It functioned as administration, not literature. The civilization collapsed around 1900 BC and the script died with it, with no bilingual key ever found.
Assessment: Fully documented. The brevity of the texts is the central puzzle.
Interesting claims
It was never writing at all
A 2004 study argued the signs lack the statistical structure of true language, perhaps a system of symbols like heraldry, not recorded speech. If so, a vast civilization ran without writing sentences.
Assessment: Fiercely contested. Computational studies have since argued both directions. Genuinely unresolved, which is rare and worth sitting with.
Stories
The lost language of the Vedas
Various claimed decipherments read the script as early Sanskrit or Tamil, each promising to rewrite South Asian history.
Assessment: None is accepted. Dozens of confident decipherments have failed independent testing.
Sources and further reading
John Marshall (ed.), Mohenjo-daro and the Indus Civilization, 1931, the seal corpus
Asko Parpola, Deciphering the Indus Script, Cambridge, 1994
Worth asking
Which is more haunting: that they had so little to say, or that they said plenty and the key died with them?