Also: about half the length of the Central and H+C lines.
Plus parts of the Circle line, Piccadilly line and East London line.
In fact, pretty much the only one that's entirely underground is the Waterloo and City...

edit: and almost all the Jubilee line north of Baker St and east of Canning Town
( Gizmo.mp3 does not exist, Fri 1 Sep 2006, 12:50, archived)