Which way do the drains run?

 Not a subject for every dinner table, but worth knowing in case you should be unlucky enough to get a blocked drain or a flood. At least the London Assembly knows which way the drains flow... The London assembly have released a draft document for sustainable urban drainage (SUDS) which contains this image of the main sewer network of London:

This plan shows a small fraction of London's invisible underground architecture, the scale it is plotted at reveals the geomorphology underlying this most urban of infrastructural developments.

It won't help the average householder very much, but