means Beneath the surface of the earth — or, figuratively, a hidden, unofficial movement or network operating outside mainstream society and law.

from A plain compound of Old English roots: 'under' (beneath) and 'grund' (ground, bottom, foundation). The literal 'below the earth' sense came first; the 'secret resistance' sense is much younger, blooming in the 20th century with clandestine political movements and wartime networks, and the 'alternative/countercultural' sense — underground music, underground press — followed close behind. Britain's railway, of course, simply went and lived there.