You changed my term, "unknown unknowns", to your own, "unknowable unknown." Obviously, I can't name an unknown unknown, since it is (you guessed it) unknown.
The change is actually quite instructive, because it says something about my understanding of man as a limited, contingent agent, and your apparent understanding of him as capable of rendering everything "worth considering" knowable.
Here's the way I see it. There are an infinitude of things to know. Obviously we won't ever know them all, but if any given thing is understandable, we can (and will eventually) understand it if only we set our minds to it. If it's fundamentally not understandable--such as the ultimate question of "why does anything exist and not just nothing"--then pondering it won't get you any closer to the truth (religious claims to the contrary notwithstanding).