where his civilizations, including our own

I think Linear A and B are somewhat better known than the Phaistos Disk. All the other undeciphered scripts that I can think of are either invented (a la the Voynich ms. or John Dee's Enochian script), or else are so early that it's debated whether they are really writing systems at all--Vinca and some other mesolithic sign systems, or even the marks found at Mont Bego and Valcamonica, which are palaeolithic, and therefore usually not considered writing systems ... but no one is really sure.

Interesting stuff, though.