For reference (warning: wikipedia), "Hampstead is the deepest station below the surface, at 58.5 metres (64 yards, 192 feet). However, it is not the deepest below sea level, as its surface building is near the top of a hill. The deepest platforms below sea level are the Jubilee Line platforms at Westminster which are -32 metres (-35 yards). The highest station is Amersham on the Metropolitan line.

[...] Only 45% of the Underground's route mileage is in tunnel: despite its name, substantial parts of the system are on brick viaducts. The highest point above ground on the Underground is the Dollis Brook Viaduct over Dollis Road between Finchley Central and Mill Hill East on the Northern Line, 18 metres (60 feet) above the ground."