In the Bible, the Land of Israel was divided into twelve areas, one for each tribe (except for the priestly tribe of Levi) . But 2,700 years ago, ten of the twelve tribes were lost, exiled by invading Assyrian armies–and nobody knows for sure where they went.
According to one legend (Targum Jonathan Exodus 34:10), the land where the tribes settled was encircled by a mystical river, the Sambatyon, which blocked them from leaving. Its rapids would only settle on the Sabbath–a day when the tribes would refuse to cross it–and so they were separated forever from the rest of the world.
Many groups–including the Beta Israel in Ethiopia, the Pashtun in Afghanistan, and even Native Americans–have claimed to be from these Ten Lost Tribes. Unfortunately, most contemporary scholars agree that it’s likely that the ten tribes just assimilated into other groups and disappeared. This position is nothing new–in the Talmud, Rabbi Akiba is quoted as saying, “Just as the day goes and does not return so [the ten tribes] went and will not return” (Sanhedrin 110b).