How far underground is the London underground?

221 feet is the greatest depth..... below are some figures to give you an idea of what depths (literally) they had to go to:

Maximum depth below mean sea level
Northern Line, just south of Waterloo station 21.3m (70ft)

Maximum depths below ground level
Deep-level tube lines
Northern Line, Holly Bush Hill, Hampstead 67.4m (221ft)

Sub-surface cut & cover lines
East London Line, Wapping 18.29m (60ft)

Average depths below ground level
Deep level tube lines 24.4m (80ft)
Sub-surface cut & cover lines 7.3m (24ft)

Deepest station
Hampstead 58.5m (192ft) below ground level

I hope you enjoy the research, cheers, Skunkfarmer - New Hampshire USA