hell vs hell on earth

hell and hell on earth both are nouns.

Word	Noun	Adjective	Verb	Adverb
hell	Yes	No	No	No
hell on earth	Yes	No	No	No

As nouns, hell and hell on earth are synonyms defined as:

Other synonyms of hell include hellhole, inferno, snake pit, the pits.

hell (noun)	hell on earth (noun)
noisy and unrestrained mischief	any place of pain and turmoil
a cause of difficulty and suffering
any place of pain and turmoil