General synonyms:
* Incredible
* Astonishing
* Amazing
* Unfathomable
* Unthinkable
* Outrageous
* Improbable
* Unlikely
* Impossible
More specific synonyms:
* Staggering (emphasizes the overwhelming nature of the event)
* Mind-boggling (emphasizes how difficult it is to comprehend)
* Unheard of (emphasizes the unprecedented nature of the event)
* Outlandish (emphasizes the absurdity or extravagance of the event)
* Preposterous (emphasizes the ludicrousness of the event)
* Fantastic (can be used both literally and figuratively to indicate extreme wonder)
The best synonym will depend on the specific context and the desired level of intensity.