River Fleet - Wikipedia

The River Fleet is the largest of London's subterranean rivers. It rises from two springs on Hampstead Heath and is directed into two water stores constructed in the 18th century, Highgate Ponds and Hampstead Ponds. From the ponds the water follows beneath the surface for 6 km to join the River Thames. The water initially flows in two paths before joining up and passing under Kentish Town and then the joined flow passes under Kings Cross, runs down Farringdon Street and joins the Thames below Blackfriars Bridge.

In Anglo-Saxon times the Fleet was a substantial body of water, joining the Thames through a marshy tidal basin over 100 metres wide at the mouth of the Fleet Valley. A large number of wells were built along its banks, some on springs (Bagnigge Well, Clerkenwell) were reputed to have healing qualtities. 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 the area was regenerated, the Fleet was converted into the New Canal, completed in 1680. Unpopular and unused the canal was filled in from 1737. The river survived slightly longer, the section from Holborn to Fleet Street 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 Kings Cross and Camden from 1812. The Farringdon Road[?] section was built over again in the 1860s with the construction of the Metropolitan Line. while the final upper section of the river was covered when Hampstead was expanded in the 1870s.