From the Wikipedia page 
The Sitovo inscription is an inscription that has yet to be satisfactorily translated. It was discovered in 1928 by an archaeological expedition led by Dr Alexander Peev on the wall of the Sitovo cave, close to Plovdiv, Bulgaria. It was most likely inscribed around 1,200 B.C. It was published in 1950 by Z. R. Morfova.
The inscription is in two lines which are 3.40 metres (11.2 ft) long. The written signs are 40 centimetres (16 in) tall.
Various attempts by both amateurs and professionals to translate it have been made. It has been identified as both ancient Celtic, Slavic and Phrygian.
- Vasil Ilyov. "DISCOVERIES ABOUT THE LITERACY, LANGUAGE AND CULTURE OF THE ANCIENT MACEDONIANS". http://www.unet.com.mk/ancient-macedonians/sitivo_a.htm. "The text is written from the right going to the left with written signs of the prehistoric Macedonian phonetic alphabet and on the ancient Macedonian language."
- Mel Copeland. "Translation of the Phrygian language". http://www.maravot.com/Phrygian5.html. "The second inscription is a rock-text near Sitovo village at 30 km south-east of Plovdiv in the Rhodopes Range. After their own autopsia Bayun-Orel 1991, 144-148, dated the inscription in III-I c. B.C. and saw on the rock "Ipta" as "figurine, cult object, image (in paredria?) with Bacchus' figurine.""
- Pavel Serafimov's reading 
See also Vinča symbols