Underground as an adjective commonly refers to something that is either below the ground or outside of public consciousness. As a proper noun, the Underground may refer to a subculture, or the Metro (mass transit system).

The underground can refer to:

• Karst topography - caves and similar
• Organized crime
• Underground culture
• UK Underground (late 1960s subculture centered around Ladbroke Grove)
• Underground resistance
• Underground hip hop
• Underground (stories)
• Underground (movie)
• London Underground
• Need For Speed: Underground (game)