* Adjective: Describing something that is located below the surface of the ground.
* Example: "The city has an extensive underground transportation system."
* Adverb: Modifying a verb, adjective, or another adverb, indicating that something is done or located below the surface of the ground.
* Example: "The tunnel runs underground for miles."
* Noun: Referring to the space below the surface of the ground, or a system of tunnels or passages below ground.
* Example: "The miners worked in the underground."
* Prepositional phrase: Used as an adjective or adverb, meaning "below the surface of the ground."
* Example: "The water pipe is located underground."
So, without more context, it's impossible to definitively say what part of speech "underground" is.