unimaginable — capable of being imagined or conceived.
unthinkable — inconceivable; unimaginable: the unthinkable size of the universe.
certain — If you are certain about something, you firmly believe it is true and have no doubt about it. If you are not certain about something, you do not have definite knowledge about it.
untenable — incapable of being defended, as an argument, thesis, etc.; indefensible.
