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

It's obvious that the black snot that you get from going down into the tube comes from the pollution down there. But this begs some questions: where does the pollution come from? And why is it worse on some lines than others? The Central line is really polluted, when you look from the platform down the tunnels there, they are grey rather than black.

While we're at it, why is it so hot down there?

It is hot on the Underground because LU don't see any need to put ventilators in. They say that you get enough ventilation from the air that is pushed through the tunnels - but it is hardly fresh, as you say, it is polluted. The air is also hot so it really doesn't help matters. The hot air actually defies the laws of physics - instead of rising up the escalator shafts, it decides to stay down underground. This is because it is mutant air - after passing through so many areas of the London Underground, it can no longer be classified as any known substance.
Hope that clears things up!

Once upon a time LU were also interested in where the black layer of grime originated from. They had a whole bunch (well one guy really) of scientists take samples of the grime and analyse it.
Turns out it's 99 percent decomposing human skin. Something to bear in mind next time you touch anything!

Why is it that some tube lines smell so different from each other?

Let's face it, one could be led blindfolded onto a tube train and be in no doubt whatsoever that it was the Northern line. Even the "lovely" new trains they've recently acquired have already taken on that burnt rubber stinkiness...