The Sign of the Cave of Pimus

Students of Rebbi Yossi ben Kisma once asked him to reveal when Moshiach would arrive. He replied, "When the arch of Rome collapses for the third time, before it can be rebuilt, Moshiach will arrive."

The students then asked for a miraculous sign to prove his words, although they had agreed beforehand not to demand proof. Rebbi Yossi answered that the water of the Cave of Pimus would turn to blood, and it did.