The Londonist has also run interesting posts on London’s lost rivers from above., and explains what the Tyburn Angling Society is up to.
These guys hope their torch batteries don’t run out…
Frank Jacobs’ Strangemaps has …… a map.
Wikipedia has a general article.
photo: Fleet River Tour by Tom Bolton, on Strange Attractor’s London leg of Obscura Day, 20 March 2010 (photo credit: Mark Pilkington)