h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Time seems to operate strangely on the Underground, which may be due to some bizarre quantum effect or perhaps London Underground have instigated time zones as well as fare zones.
The Northern Line, as the most difficult to navigate, also shows this effect most strongly. It can be helpful when using this line to think in terms of Northern Line minutes rather than any international unit of time.
Mostly Northern Line minutes are longer than standard minutes although occasionally they can be four or five times shorter. This effect tends to transpire when you nip back to the street for a paper or coffee, thinking that you have suffiecient time.
Northern Line minutes are longest when you are tired and want to get home or during the morning rush when the platform is three or four people deep.
Occassionally they have to inject leap minutes to maintain the effect. This usually occurs when you are in the middle of a tunnel on a hot day with a mad person in the carriage and can involve stopping for several normal minutes. However, for those waiting at the next station Northern Line minutes will have slowed so that the train still arrives on time.
Journey times are nearly always an hour whereever you're going so perhaps time on the Underground is immaterial anyway.

I think in several thousand years time archeologists will still be attempting to work out what the underground maps are(I'm still trying to work out how they work and I live in the right time zone), and why a civilisation dug so many tunnels. They will probably theorise that we worshipped a kind of underground, alien god where we needed to descend various levels before reaching a state of Covent Garden.