fathom vs understand

fathom is a noun but understand is not a noun.

Word	Noun	Adjective	Verb	Adverb
fathom	Yes	No	Yes	No
understand	No	No	Yes	No

As verbs, understand is a hypernym of fathom; that is, understand is a word with a broader meaning than fathom:

fathom (noun)	understand (noun)
a linear unit of measurement (equal to 6 feet) for water depth
(mining) a unit of volume (equal to 6 cubic feet) used in measuring bodies of ore

fathom (verb)	understand (verb)
measure the depth of (a body of water) with a sounding line	know and comprehend the nature or meaning of
come to understand	perceive (an idea or situation) mentally
	believe to be the case
	make sense of a language
	be understanding of