Somewhere beyond the known world, the sages said, there runs a river that refuses to behave like a river. It is called the Sambatyon, and it does not flow with water. It rushes with stones.
Six days a week the Sambatyon hurls pebbles and boulders down its course with such violence that no boat can cross and no man can ford it. But on the seventh day — on Shabbat — the river falls silent. The stones rest where they lie. The banks are quiet. And the water, such as there is, runs smooth as glass until the stars come out.
The proof of Shabbat's cosmic reality, the rabbis argued in Sanhedrin 65b, lies in this strange river. A necromancer was once confronted with his failure to raise the dead on the seventh day. Rashi adds that the Sambatyon also proves the point — a river that keeps Shabbat is a river that answers to the same law as Israel.
The Pentecost Machzor calls it the incomprehensible river. Yalkut on Isaiah and Pesikta Tanchuma both preserve the tradition. The Ten Lost Tribes, some said, live on the far side of the Sambatyon, unable to return because the river will not let them pass except on a day when they themselves must not travel.
It is a riddle wrapped in a geography lesson. The world itself, according to the rabbis, takes one day off a week — and that is the day when the exiles cannot come home.