There are a plethora of synonyms that can be used in place of the phrase 'is unbelievable'. Such synonyms can include 'astonishing', 'amazing', 'incredible', 'unimaginable', 'mind-blowing', 'breathtaking', 'staggering', 'miraculous', 'overwhelming', 'stupefying', and 'jaw-dropping'. These words are often used to convey the sense of disbelief or amazement that arises when encountering something that is truly remarkable or unexpected. Whether it be a momentous event or an incredible feat, the power of language can help to convey the significance of such occurrences. By utilizing synonyms, one can add variety, emphasis, and deeper meaning to their writing, ensuring that their message is clear and impactful.