Meaning: when the impossible or improbable happens
Rewrite 1: when hell floods out
Rewrite 2: when heaven welcomes Satan
Rewrite 3: when the sun shuts down
Rewrite 4: when ice flows in the Amazon

Comment: You can be creative here, but it becomes unbelievable if you push the boundaries too far. Also, heaven and hell are often but not always capitalized.

More reading about clichés
What I found when I googled “clichés”:
• Clichés – English (ESL) Weblog: “A cliché is a group of words that have special meaning…”