Undeciphered writing systems
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Many undeciphered writing systems date from several thousand years BC, though some more modern examples do exist. The term "writing systems" is used here loosely to refer to groups of glyphs which appear to have representational symbolic meaning, but which may include "systems" that are largely artistic in nature and are thus not examples of actual writing.
The difficulty in deciphering these systems can arise from a lack of known language descendants or from the languages being entirely isolated, from insufficient examples of text having been found and even (such as in the case of Vinča) from the question of whether the symbols actually constitute a writing system at all. Some researchers have made claims of being able to decipher certain writing systems, such as those of Epi-Olmec, Phaistos and Indus texts; but to date, these claims have not been widely accepted within the scientific community, or confirmed by independent researchers, for the writing systems listed here (unless otherwise specified).
Certain forms of proto-writing remain undeciphered and, because of a lack of evidence and linguistic descendants, it is quite likely that they will never be deciphered.
- Jiahu symbols — Peiligang culture, from the 7th millennium BC.
- Vinča symbols — Neolithic Europe, from the 6th millennium BC.
- Dispilio Tablet — Neolithic Europe, from the 6th millennium BC.
- Banpo symbols — Yangshao culture, from the 5th millennium BC.
Bronze Age scripts 
The following is a list of undeciphered scripts from the Bronze Age (3300 to 1200 BC).
- Indus script — Indus Valley Civilization, proto-writing from ca. 3300 BC, mature script ca. 2500-1900 BC.
- Proto-Elamite — Elam, from ca. 3200 BC.
- Linear Elamite, from ca. 2200 BC.
- Linear A, from ca. 1900 BC, a syllabary.
- Cretan hieroglyphs, from ca. 1900 BC.
- Linear A and Cretan hieroglyphs are both believed to be an example of the Minoan language. Several words have been decoded from the scripts, but no definite conclusions on the meanings of the words have been made (with the exception of kuro, meaning "total").
- Wadi el-Ħôl script, ca. 1800 BC, likely an abjad.
- Byblos syllabary — the city of Byblos, ca. 1700 BC.
- Phaistos Disc, ca. 1600 BC, a unique text found on one single object; a short inscription on the Arkalokhori Axe possibly represents the same type of writing.
- Cypro-Minoan syllabary, from ca. 1500 BC.
- Southwest Paleohispanic script, from ca. 700 BC.
- Sitovo inscription, probably Phrygian.
Mesoamerican scripts 
Many Mesoamerican writing systems have been discovered by archaeologists. Many of them remain undeciphered due to a lack of knowledge of the original language. These writing systems were used between 1000 BC and 1500 AD.
- Olmec — Olmec civilization, ca. 900 BC, possibly the oldest Mesoamerican script.
- Isthmian, ca. 500 BC, apparently logosyllabic.
- Zapotec — Zapotec, ca. 500 BC.
- Mixtec — Mixtec, 14th century, perhaps pictographic.
South American scripts 
- Quipu — Inca Empire, 15th century, is thought by some to have been a writing system, but is generally believed to be an accounting system.
Medieval and later scripts 
- Alekanovo inscription
- Issyk writing (ancient Turkestan and Afghanistan)
- Khitan scripts — Khitan, 10th century, not fully deciphered.
- Tujia script
- Singapore Stone, a fragment of a sandstone slab inscribed with an ancient Southeast Asian script, perhaps Old Javanese or Sanskrit. At least 13th century, and possibly as early as 10th to 11th century.
- Rongorongo — Rapa Nui (aka Easter Island), before 1860.
Related concepts: texts that are not writing systems 
One very similar concept is that of false writing systems, which appear to be writing but are not. False writing cannot be deciphered because it has no semantic meaning. These particularly include asemic writing created for artistic purposes. One prominent example is the Codex Seraphinianus.
Another similar concept is that of undeciphered cryptograms, or cipher messages. These are not writing systems per se, but a disguised form of another text. Of course any cryptogram is intended to be undecipherable by anyone except the intended recipient so vast numbers of these exist, but a few examples have become famous and are listed in the uncracked codes and ciphers category.
Possible hoax undeciphered writing systems 
- Voynich manuscript, estimated to have been created circa 1450-1520, based on illustrations contained within the manuscript. Recent carbon dating has dated it to the 15th century. In terms of provenance, the earliest confirmed references to the work date only to the early 17th century.
- Rohonc Codex, before 1838.
- Proto-Elamite (CDLI link)
- Austin Simmons, The Cipherment of the Franks Casket
- Vinča signs (The Old European Script: Further evidence - Shan M. M. Winn)