Rabbi Shimon bar Yochai was so great that, during his lifetime, no rainbow ever appeared in the sky over the Land of Israel.
The rainbow, in rabbinic tradition, is not only a covenantal memory from Noah's flood (Genesis 9:13). It is also a warning signal. When the generation grows so wicked that a new flood would be justified, heaven hangs a rainbow in the sky as a reminder that the Holy One has promised not to destroy the world. The rainbow appears because mercy is having to work overtime.
As long as Rabbi Shimon bar Yochai was alive, no such warning was needed. His merit alone — his Torah, his righteousness, his years hidden in the cave with his son Elazar — held up the generation like a column. Calamity could not enter while he stood.
One day the prophet Elijah and Rabbi Joshua ben Levi approached him. Rabbi Shimon embraced Elijah but refused to look at Rabbi Joshua. When pressed for a reason, he said: "During your lifetime a rainbow was seen. That means your generation deserved to be destroyed and was saved only by the covenant. Your merit was not sufficient to cover them" (Ketubot 77b; Gaster, Exempla No. 205).
The Ma'aseh Book records other wonders about Rabbi Shimon bar Yochai — once, the tradition says, he commanded a barren valley, "Be full of gold," and the valley filled.
The story is astonishing and unsettling. It teaches that the righteous hold up the world in ways we cannot measure, and that a single saint's existence can be the reason a generation does not see the warning signs.