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The Sambatyon is a mythical river separating the Ten Lost Tribes from the rest of Israel. The first mention is in the Targum Yonatan on Exodus 34:4, "I will remove them from there and place them beyond the River Sambatyon."
Rabbi Akiva remarks on the river's Sabbath observance when he tells his Roman interlocutor Ternius Rufus that the Sambatyon ceases to flow on Shabbat (Genesis Rabba 11).
There is some argument about the Sambatyon's location. Josephus places it in Syria, Nachmanides names Media, and others suggest India or Ethiopia.
The Middle Ages were the heyday of the Sambatyon, with both Jews and Christians seeking it out to find the Lost Tribes. A ninth-century Jewish traveller named Eldad Hadani, who claimed to be from the kingdom of the tribe of Dan, described the Sambatyon as a waterless river of rocks and sand.
Finding the Sambatyon was understood to be a means of hasten the coming of the Messiah through reuniting with the Lost Tribes. Today, it has mythical connotations of a lost world and hope for the future.