Deepdown is a word that means "at the very core of something". There are many synonyms for this word that can be used to express the same idea. Some of these synonyms include "innermost", "profound", "core", "fundamental", "essential", "intrinsic", "rooted", "central", "basic", "crucial", "elemental", "vital", and "inmost". All of these words can be used in place of deepdown to convey a sense of the underlying, fundamental nature of something. Whether describing a person's beliefs or the nature of a problem, these synonyms can help to convey a sense of the deep-seated importance of the matter at hand.