A rich source is a plentiful and abundant source of something valuable. The term can be broken down into three crossword solutions: FOUNT (5 letters), LODE (4 letters), and MINE (4 letters). A FOUNT is a natural spring or fountain, a LODE is a vein or deposit of ore, and a MINE is an excavation in the earth from which minerals are extracted. All three terms refer to a place where something valuable can be found in abundance.