From the Wikipedia page on Proto-Elamite .
The oldest known undeciphered script.
The Proto-Elamite period is ca. 3200 BC to 2700 BC when Susa, the later capital of the Elamites, began to receive influence from the cultures of the Iranian plateau.
The Proto-Elamite script is an Early Bronze Age writing system briefly in use for the ancient Elamite language before the introduction of Elamite Cuneiform.
Although Proto-Elamite remains undeciphered, the content of many texts is known. This is possible because certain signs, and in particular a majority of the numerical signs, are direct loans from the neighboring Mesopotamian writing system, proto-cuneiform. In addition, a number of the proto-Elamite signs are actual images of the objects they represent. However, the majority of the proto-Elamite signs are entirely abstract, and their meanings can only be deciphered through careful graphotactical analysis.
While the Elamite language has been suggested as a likely candidate underlying the Proto-Elamite inscriptions, there is no positive evidence of this. The earliest Proto-Elamite inscriptions, being purely ideographical, do not in fact contain any linguistic information, and following Friberg's 1978/79 study of Ancient Near Eastern metrology, decipherment attempts have moved away from linguistic methods.
In 2012, Dr Jacob Dahl of the Faculty of Oriental Studies, University of Oxford, announced a project to make high-quality images of Proto-Elamite clay tablets and publish them online. His hope is that crowdsourcing by academics and amateurs working together would be able to understand the script, despite the presence of mistakes and the lack of phonetic clues. Dahl assisted in making the images of nearly 1600 Proto-Elamite tablets online
A BBC article on decipherment is here