The only sign Jesus promised his generation was a prophet three days in the deep.
Read the verse in full at Blue Letter Bible (ESV).
The first taste — one dip, the whole day.
Asked for a sign, Jesus offered exactly one: 'as Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth.' Jonah went down into death's territory for his own rebellion; Jesus went down for ours. Jonah emerged to preach one reluctant sermon that saved a pagan city; Jesus emerged to commission a gospel for every city.
Promise and fulfillment — every claim linked for verification.
Jesus adds the comparison that matters: 'something greater than Jonah is here' (Matt 12:41). Nineveh repented at a runaway's sermon; how much more the risen Son's?
'Sign' — not a spectacle but a signpost, an act that points beyond itself. Jesus refused signs-as-entertainment and staked everything on one sign-as-verification: the resurrection (Matt 12:39–40).
Deepest water — where the passage meets your week.
Jesus concentrated the credibility of his entire claim into one testable event. That is either recklessness or certainty. It also simplifies your apologetics: the faith does not stand on every question being answered but on one tomb being empty. Keep your own confidence — and your conversations with skeptics — anchored where Jesus anchored his.