Some interesting facts about the London Underground:

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

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

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

Longest continuous tunnel -Northern Line, East Finchley to Morden (via Bank) 27.8km (17.4 miles)

Longest distance between stations -Metropolitan Line, Chesham to Chalfont & Latimer 6.26km (3.91 miles)

Highest point above mean sea level -Metropolitan Line, beyond Amersham Station 150m (492ft)

Highest station above mean sea level -Amersham 147m (482ft)