This term is often associated with catastrophe. The unimaginable destruction of a tsunami for instance. I prefer to view it in a more positive light. The most amazing and awe inspiring sights and insights can be unimaginable in our day to day lives. We think of them as isolated and maybe even "once in a life time" occurrences.
I think they happen far more frequently. These divine whisperings often go unheard because we are too busy attending to more "important" things. Perhaps we don't think "divine whisperings" are meant for ordinary ears, as if we are not worthy somehow. How very wrong that thinking is.
I am reading a small book about the "Showings" of Julian of Norwich, the 13th century English mystic. She gave voice to many unimaginable ideas for her time and had no problem hearing the divine whispers. I think, however, that she would reject the thought that these" unimaginable whisperings" are either unique or exclusive to mystics. As Carl Jung advises,
The soul is an entity endowed with the consciousness
of its relationship to deity.