underground

Underground describes something located beneath the surface of the earth. In geology, archaeology, and engineering, it refers to spaces, structures, or features that are not exposed to the ground level. The term is also used figuratively to describe activities or networks that operate out of sight or outside the mainstream.

In transportation, the term is commonly used for urban rapid transit systems that run mostly below street level. The London Underground, nicknamed the Underground, is the oldest such system and has become a symbol of London’s transport network. Other cities use similar terms such as subway or metro.

Underground mining involves extracting minerals from beneath the surface, typically through tunnels and shafts. Methods include room-and-pillar and longwall mining; it presents safety risks and environmental impacts that require specialized engineering and monitoring.

The word is also applied to clandestine networks and movements, including resistance groups, espionage networks, or the historical Underground Railroad in the United States, which helped enslaved people escape to free states and Canada in the 19th century.

In culture and economy, “underground” can denote activities outside the formal mainstream, such as the underground economy, underground music and arts scenes, or the underground press that operated outside mainstream media. The term also appears in references to subcultures and phenomena that are not part of the dominant public sphere.