effort to decode the carvings made by an Iron-Age Celtic people known as the Picts. The Picts were around between 300 and 800 A.D. The rather wonderful stone patterns and images could be either just that - nice pictures with symbolic importance - or possibly a real, complex, written language. No one knows, but a careful statistical analysis of the arrangement of the Pictish pictures now suggests that they are consistent with a real writing system. That only leaves one problem - how to decipher it ! At the risk of sounding like a naysayer, if it's this hard to interpret the handiwork of our own species, just imagine how hard it will be to deal with an unknown alien species.
Of course, the Picts didn't (as far as we know) use complex mathematics, or have a detailed knowledge of the physics of nature. Those could indeed serve as a common ground, although one wonders how easy that would be - does an alien description of atoms or quantum mechanics look like ours ?