A sign . . . a sign. Well, let's see now, they just witnessed a glorious manifestation of heaven exploding onto earth, and they had pronounced it an act of sorcery from hell. Has anyone else in the history of mankind seen the wonders they have? People in times past have seen far less and believed. The men of Ninevah heard the preaching of Jonah and immediately repented in sackcloth and ashes, yet Jonah worked no wonders before their eyes, no miraculous healings. The Queen of Sheba traveled from afar on the basis of mere hearsay to listen to the wisdom of Solomon, and she believed though she saw no lepers healed, no demons cast out. What's more, these people were Gentiles. Somehow they got more faith-mileage out of crumbs falling from Israel's table than the Pharisees and scribes who occupy privileged seats at the banquet of God's glory. Yet they complain that they lack a sign. Really?
You can see why Jesus calls them "an evil and adulterous generation." Not adulterous literally but spiritually: they are unfaithful to Yahweh and that has left them spiritually blind. No matter what sign is performed before their eyes they will never perceive its meaning, so the only one they will be given is the sign of Jonah. Just as Jonah was in the belly of the sea monster for three days and three days, so the Son of Man will lie in the belly of the earth and then be raised to life again.
Why is Jonah's time in the fish's belly like Jesus' time in the grave? Jonah couldn't have been closer to death than to be in that fish's stomach waiting to be consumed by its digestive organs. In his prayer (Jonah 2:1-9) he describes himself as in "the depths of Sheol," "descended to the roots of the mountains," and says that God "brought up my life from the pit." Jonah's "resurrection" isn't nearly as glorious as Jesus'--the fish barfed him up onto dry land--but otherwise his harrowing experience does closely parallel Jesus' three-day-long embrace in the arms of death. Reading the prayer in Jonah 2 makes you wonder if the emotions Jonah expresses were also Jesus' during his own frightening descent into death's maw.
Jesus' death and resurrection would be the only sign that the Pharisees and scribes would receive from here on out. The sign of Jonah. For countless others it would be the only sign needed to believe, but for the proud and hard-hearted it would serve only to condemn them further on the Day of Judgment, when witnesses from Ninevah to Sheba would rise up and testify that they had been given far less revelation and yet believed.