The word "un-fathomed" refers to something that is not fully understood or explored. Its antonyms would be words that describe exactly the opposite - things that are well-known and comprehensible. Some possible antonyms for "un-fathomed" include "comprehensible," "understood," "known," "familiar," "clear," and "transparent." Other antonyms could be "unambiguous," "evident," "simple," "straightforward," or "uncomplicated." These words suggest a level of knowledge and understanding that is complete and easily graspable. Unlike "un-fathomed," they imply a sense of order and certainty, rather than mystery and abstraction.