the word underground is a common noun but if used like Underground Railroad then it is proper
"Underground river" is a common noun that refers to any river that flows beneath the surface of the Earth. It is not a specific or proper noun unless referring to a specific, named underground river such as the Puerto Princesa Underground River in the Philippines.
"Park Avenue" is a proper noun, because it is a place. Proper nouns like this should always be capitalized.
Common
It is a proper noun, because it is the name of a specific thing.
proper
Proper noun
Pencil proper or common noun
it's a common noun. a proper noun would be Spider-Man.
Proper noun or common noun
The noun cassette is a common noun.
a common noun?
The answer is proper noun. Examples of common noun and proper noun are: Proper noun: Mary Collins Common noun: monkey