What is a Sentence using unfathomable?

Answer 1

It is unfathomable that somebody jumped off a building and survived.

----That is good----- NOUN

The unfathomable boy tried hard to explain his side of the story to the jury, but it looked as though they were in disbelief. ADJECTIVE

----that is better-----

Answer 2

"My teacher asked me to write a sentence using the word 'unfathomably'." is always a good option.

The fathom, is, of course, a noun, a measure of water depth, equal to 6 feet. It spawned the verb, 'to fathom', to measure the depth of water, and a adjective pair, 'fathomable' and 'unfathomable', to describe water that was to deep to be measured with a weight and line.

From there, the word 'fathom' came to be used as 'to understand' a difficult or complex thing. It's a simple metaphor.

What you have is an adverb, drawn from the adjectives, which is generally paired with a word such as 'great', 'deep', or 'complex': words that are themselves synonyms of 'unfathomable'. The reason for it is that we sometimes want to use more words than we need to. (See this answer for an example!)

So, my final suggestion: "'Unfathomably' is an overly complex word that adds very little to any sentence."