Warning: video and photo gallery contain images that might put you off your lunch.
The River Fleet rises on Hampstead Heath, flows down through Kentish Town, Camden and King's Cross to Blackfriars. You've probably never seen it. For the past couple of hundred years it's been hiding beneath ground in a sewer. We recently took a paddle through the buried watercourse in the company of Tom Bolton, author of London's Lost Rivers.
With thanks to Thames Water for granting access and supervision.