The bad-figs continuously requested signs from Immanuel-Christ-Jesus. Immanuel = GOD with us: came from eternity, into time to dwell among us; and to meet each of us where we were - to get us where we all need to be. GOD is Good!
In Matt.12:38 we have some of the fig-tree-generation - "we would see a sign from Thee." Immanuel-Christ-Jesus identified this group in Vs.39, and brought the Prophet Jonah on as a Witness and reminder: the same Religion[Traditions of men] was involved now, as was in Nineveh.
The Prophet Jonah [Jonah1] received instructions to go to the wicked city, Nineveh: He went the other way. On board a ship the wind and waves were sinking the ship and the sailors id Jonah as the curse. After tossing him overboard a great-fish swallowed Jonah. The BIG Sign was Jonah was dead - for three days, and nights, in the belly of the fish[Matt.12:40 requires his death].
Because of Jonah's journey and mode of transport, ALL of the people of Nineveh REPENTED! The LORD was able to convert the sailors as well. The Prophet Jonah was a "type" of Christ. Via his death, many lives were Saved.
Nineveh is ref. in Part 1 of the Bible going back approx. 6,000 yrs.. Approx. 3,000 yrs. ago it had become an important Religious-center for worship of the Akkadian goddess Ishtar[Easter]. One of the symbols employed is the fish: it is not by chance that Jonah was transported by a great-fish, or Christ had a fish deliver money in its mouth!