underground
Below the ground; below the surface of the Earth.
Example
“There is an underground tunnel that takes you across the river.”
Other meanings
hidden Hidden, furtive, secretive.
“These criminals operate through an underground network.”
Of or relating to an art forms (such as music) or subculture that is outside the mainstream, especially one that is unofficial and hidden from the authorities.
“underground music”
Word history
From Middle English "undergrounde".
Where this comes from
This definition is adapted from “underground” on Wiktionary, by its contributors, published under CC BY-SA 4.0. This page is published under the same licence.