Underground
Part of speech: adverb
Definition: Below the ground. Secretly.
Part of speech: adjective
Definition: Below the ground; below the surface of the Earth. Hidden, furtive, secretive. Of music, art, etc, outside the mainstream.
Part of speech: noun
Definition: An underground railway. (with the) A movement or organisation of people who resist political convention. (with the) A movement or organisation of people who resist artistic convention.
Example sentence: The work an unknown good man has done is like a vein of water flowing hidden underground, secretly making the ground green.
Metro
Part of speech: noun
Definition: An underground railway other than those in the US or London, in particular that in Paris.A train that runs on such an underground railway.An urban rapid transit light railwayA train that runs on such a railway.
Example sentence: People across metro Detroit face discrimination every day in housing, employment, insurance - the list goes on. It might not always be explicit and in your face, but my residents know when they're being mistreated.