Ineffable is an adjective used to describe something that is so overwhelming that it's unspeakable or indescribable.
Aside from those more obvious considerations touching Moby Dick, . . . there was another thought, or rather vague, nameless horror concerning him, which at times by its intensity completely overpowered all the rest; and yet so mystical and well nigh ineffable was it, that I almost despair of putting it in a comprehensible form.
From Herman Melville's Moby-Dick