The highest point reached by tube trains is at Amersham on the Metropolitan Line at about 500 feet above sea level. However the highest point above the ground is reached on the Dollis Brook Viaduct on the Mill Hill East branch of the Northern Line which is 60 feet above the road below.
The lowest point on the system is just south of Waterloo station, where the tracks are 70 feet below sea level. The absolute deepest part of the system is also on the Northern where, below Hampstead Heath, the rails are over 220 feet below the ground.
A steam train which carried passengers in the 19th Century has returned to mark 150 years since the first tube journey.
A look back at some of the stops on London Underground's 150 year journey.
All day we've been adding fascinating facts about the Tube, which turns 150 years old this week. Now find out how much you know.