In any case, the FT has a wonderful article on another hidden river, The Fleet in London:
The Fleet became a noxious ditch and, in 1679, the build up of filth burst under the pressure of the water behind it and washed away several butchers around Smithfield meat market, cattle and all. The poet John Gay, perhaps employing his profession’s licence, thought the river delightful and observed: “Fleet Ditch with muddy current flows.” Gay also recommended the oysters for sale on the quayside. Such shellfish would struggle to pass health and safety muster now. Much better to stop at The Eagle, the bar that started Britain’s gastropub revolution, and which tempts me in with those ancient London scents: meat, beer and fish.
It really is a wonderfully evocative piece and well worth reading. If you are looking for something more, you could read the Wikipedia article linked above or you could check out Mr. J Doe’s Flickr stream (his image adorns this post) and read some the fascinating captions he has on the sequence of photographs of London’s Sewers.