undergroup

Underground refers to anything situated, conducted, or existing beneath the surface of the earth. This can encompass a wide range of phenomena, from natural geological formations to human-made structures and activities.

Geologically, underground refers to the subsurface layers of the Earth's crust, including soil, rock, and subterranean water systems. This includes caves, aquifers, and mineral deposits. The study of these underground environments is the domain of geology and hydrogeology.

In a human context, "underground" often describes infrastructure such as tunnels, subways, and utility lines like water mains and sewer systems, which are built below ground level for practical reasons of space, safety, or aesthetics. It can also refer to concealed or clandestine operations, such as underground economies or resistance movements, which operate outside of official or legal recognition. The term can also be used metaphorically to describe subcultures or artistic movements that exist outside the mainstream.