The people of Nineveh believed God; and they proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them.
"Jonah became a sign to the Ninevites ..." This is the only proof needed that the Ninevites were fully aware of the supernatural wonder involved in Jonah's deliverance.  Here again, we have that great New Testament word used also in  John 2:11, etc., which "denotes a miraculous act, given as a token of divine power and authority."[23]  The Scriptures do not tell us "how" Nineveh learned about this, but they do clearly inform us that they did learn of it.  As to the "how"; it may be assumed that Jonah himself related his experience with God's anger, and with God's punishment, and following his repentance, with God's mercy.  If Jonah did not himself tell, there were other witnesses, the mariners; at any rate there was ample human testimony.