When one of the Lamed Vav dies, another is immediately chosen to take his place. Often, the Tzadik Nistar does not even know he is chosen for the task. As long as the Lamed Vav continue to serve humanity and God in this fashion, the world will go on. But if at some point God will not be able to find someone just and good enough to replace a dying Tzadik, the world will end immediately.
The Lamed Vav are two letters in the Hebrew alphabet. Numerically, they represent 36. Legends tell that in this world, there always live thirty-six men who are also called Tzadikim Nistarim, or the Hidden Just Men. They are usually poor, unknown, obscure, and no one guesses that they are the ones who bear all the sorrows and sins of the world. It is for their sake that God does not destroy the world even when sin overwhelms mankind.