The Heart of London...

I am not talking about a particular area of the capital city, but a huge part of London's infrastructure, without which, the city would - literally - grind to a halt.  I am, of course, talking about the London Underground system, or as we call it, "the tube".  I know Londoners are quite happy to complain about the overcrowding, delays etc, but I really don't know what they would do without it; I know I miss it...