The Krop Tor script was an ancient writing system for an unknown language. The script was found on old pieces of pottery and a notebook, found on Krop Tor by the Galis Expedition. (WC: Tardisode 8, TV: The Impossible Planet) The language was so old that not even the Doctor's TARDIS was able to translate it. (TV: The Impossible Planet)
The script was also found on Toby Zed's skin when he was being possessed. (TV: The Impossible Planet, The Satan Pit)