Where is the Sambatyon? Is there really a river somewhere that throws stones all week and rests on Shabbos?
Well, an encyclopedia entry on it says the earliest source is Pliny's Historia Naturalis, in which a river with similar miraculous properties is in fact reported in XXXI:18 (last line). The footnote there identifies it, citing a later source, as the river Eleutherus, which is currently in northern Lebanon.
I once saw an article (don't recall where) that identified it with the Dardanelles, which separates Europe from Asia. Part of the author's idea was that it was originally called ים בטיון ("Sea of the Bithynians," an ancient people who lived in that area), and that got corrupted to סמבטיון.
A contemporary traveler claimed (2011) that the "Sambatyon" is located in Nepal, near India, and that he was once there.
According to his account, it's not a river of water, rather a mountain that throws stones. The name "river" is a borrowed term, as water does not flow in it, but has a similar essence in that it roars, makes noises, and hurls sand, rocks, and boulders, akin to a river raging in its waters.
He reported that at sunset on Friday the activity totally stopped, and the silence continued until the conclusion of Shabbos.