The River Fleet in London is a tidal river that once provided water for many industries. Over the years, it became quite polluted, then was consigned to flow underneath the city as London grew, until it was eventually incorporated into the sewer system. But the river is still there, filling its tunnels at high tide and ebbing to a trickle at low tide. Read about what happened to the River Fleet and see plenty of pictures at Kuriositas. Link

(Image credit: Flickr user sub-urban.com)