The hidden city
This light-hearted map, originally produced for Heritage magazine, charts the secrets under London's streets in the style of Frank Beck's famous tube map. It has since been taken up by Metro, the Independent on Sunday and thetube.com.
Illustrative rather than definitive, it includes the (now-closed) Post Office Railway, a selection of the capital's buried rivers, Joseph Bazalgette's sewer system and some of the curiosities of the Northern Line.
In a similar style is a Beck-style map of London's canals and navigable rivers, currently not publically available pending discussions between British Waterways and Transport for London.
See the map
You can view and download the Underground London map here, in one of three sizes: