The River Fleet is the largest of London's subterranean rivers. It formerly flowed on the surface. It rises from two springs on Hampstead Heath and is directed into two reservoirs constructed in the 18th century, Highgate Ponds and Hampstead Ponds. From the ponds the water flows underground for to join the River Thames.
In Roman times, the Fleet was a major river, with a tide mill in its estuary. In Anglo-Saxon times, the Fleet was still a substantial body of water, joining the Thames through a marshy tidal basin over wide at the mouth of the Fleet Valley. A large number of wells were built along its banks, and some on springs (Bagnigge Well, Clerkenwell) were reputed to have healing qualities. As London grew, the river became increasingly a sewer. By the 13th century, it was considered polluted, and the area was given over to poor-quality housing, and, later, prisons (Newgate, Fleet and Ludgate prisons were all built in that area). The flow of the river was greatly reduced by increasing industry.
Following the Great Fire of London in 1666, Christopher Wren proposed widening the river; however, this was rejected. Rather, the Fleet was converted into the New Canal, completed in 1680. Newcastle Close and Old Seacoal Lane (now just short alleyways off Farringdon Street) recall the wharves that used to line this canal, especially used by the coastal coal trade from the North East of England. Unpopular and unused, the canal was filled in from 1737. The river survived slightly longer: The section from Holborn to Ludgate Circus was channelled below the surface when the canal was filled, with the section to the river covered by 1765. The development of the Regent's Canal and urban growth covered the river in King's Cross and Camden from 1812. The Farringdon Road section was built over again in the 1860s with the construction of the Metropolitan Railway, while the final upper section of the river was covered when Hampstead was expanded in the 1870s.
The river gives its name to Fleet Street which runs from Ludgate Circus to The Strand. In the 1970s, a planned London Underground tube line was to run alongside the line of Fleet Street. However the route was changed to go south of the river and in honour of the Queen's Silver Jubilee in 1977, the name was changed from Fleet Line to Jubilee Line.
The Fleet can be heard through a grating in Ray Street, Farringdon (EC1) in front of the Coach and Horses pub. The position of the river can still be seen in the surrounding streetscape with Ray Street and its continuation Warner Street lying in a valley where the river once flowed. It can also be heard through a grid in the centre of Charterhouse Street where it joins Farringdon Road (on the Smithfield side of the junction).
The Mayor of London has proposed opening up short sections of the Fleet and other rivers for ornamental purposes.