Despite being called the London Underground, more than half of the system is in fact above ground (some of it a long way above ground- Amersham station is 147 metres above sea level). However, the feeling of descending into the bowels of the earth will be familiar to most Londoners. It'll be particularly familiar to any passengers who frequent Hampstead station, which is the deepest one on the system at 58.m metres below the surface. It's amazing there's not been an epidemic of the bends in north London.