Which London Underground line is the deepest?
My answer to Which London Underground line is the deepest? on Quora
The Northern line has both the deepest point below ground (220 feet, at Hampstead) and the deepest below sea level (70 feet, south of Waterloo)
Update: Dave Arquati corrected me in a comment: since the 1990 Jubilee Line extension “the deepest point below sea level is now on the Jubilee line near Waterloo (105ft).”