unfathomable
adj.adj. so difficult to understand that you cannot see the reason for it. You use this when something is completely confusing or mysterious.
adj. impossible to understand or explain; beyond the reach of human comprehension. Often used to describe complex problems, deep emotions, or vast distances.
The reason for his sudden change was unfathomable to everyone.
The depth of the ocean is unfathomable, with many parts still unexplored by modern technology.
The sudden collapse of the bridge was an unfathomable tragedy that left the entire community in shock and demanded a complete review of safety standards.
From un- (prefix meaning ‘not’) + fathom (“to measure the depth of (water); (figurative) to deeply understand (someone or something)”) + -able (suffix forming adjectives denoting things not able or fit to be done).