The University of Cambridge news service has a story (no author credit given) about an endangered dialect of Pontic Greek called, for some reason, Romeyka (a general Greek term for Modern Greek; the local name for the dialect, according to Wikipedia, is Rumca, ‘the language of Rûm,’ which is also the original meaning of Romeyka). The story, being the product of a publicity department, is overhyped (the lead implies it’s some sort of new discovery, whereas it’s been known for ages, and in fact the researcher, Dr. Ioanna Sitaridou, was told about it by Peter Mackridge, who’d written about it back in 1987) and occasionally misleading (they call the infinitive “the basic, uninflected form of the verb”), but it’s got enough interesting material to be worth a read. An excerpt:
“Although Romeyka can hardly be described as anything but a Modern Greek dialect, it preserves an impressive number of grammatical traits that add an Ancient Greek flavour to the dialect’s structure – traits that have been completely lost from other Modern Greek varieties,” Dr. Sitaridou said. “What these people are speaking is a variety of Greek far more archaic than other forms of Greek spoken today.” …
Despite millennia of change in the surrounding area, people in the isolated region still speak the language. One reason is that Romeyka speakers are devout Muslims, and were therefore exempt from the large-scale population exchange between Greece and Turkey that took place under the Treaty of Lausanne in 1923.
There’s also a brief video where you can hear a few snippets of the dialect (and see some gorgeous scenery).
Addendum. An excellent discussion, with appropriate debunking, at Hellenisteukontos.