Underground tunnels are a location or area where a maze of shafts have been carved through the earth. They may be hidden tunnels, sapping tunnels, maintenance tunnels, sewer tunnels, mining tunnels or tunnels for some type of underground vehicle. They may be naturally occurring cave systems, or dug by humans or creatures. They may be used to connect above ground locations or areas of an underground city. Tunnels are often used as an escape route. They may include caved in portions, or flooded portions.

When designing underground tunnels consider the source of light and how they are supported. Consider where and how they may connect to the surface.

Tunnels can be used as a terrain feature in combat-oriented game worlds to provide cover and create flanking opportunities. In narratives, they often are an allegory for death and the afterlife.