Fleet river

The River Fleet is the largest of London's subterranean rivers. Its headwaters are two streams on Hampstead Heath, each of which was dammed into a series of ponds, the Hampstead Ponds and the Highgate Ponds, in the 18th century. At the southern edge of Hampstead Heath these descend underground as sewers and join in Camden Town. The waters flow 4 mi (6 km) from the ponds to the River Thames.

The river gives its name to Fleet Street, the eastern end of which is at what was historically the crossing over the river known as Fleet Bridge, and is now the site of Ludgate Circus.

The Fleet arises on Hampstead Heath as two sources, which still flow on the surface as the Hampstead Ponds and the Highgate Ponds. Then they go underground, pass under Kentish Town, join in Camden Town, and flow onwards towards St Pancras Old Church, which was sited on the river's banks. From there it passed in a sinuous course which is responsible for the unusual building line adjacent to King's Cross station; the German Gymnasium faced the river banks, and the curve of the Great Northern Hotel follows the river which passes alongside it.

King's Cross was originally named Battle Bridge, referring to an ancient bridge over the Fleet where Boudica's army is said to have fought an important battle against the Romans; the name was changed in the 19th century, to refer to a statue of George IV, widely disliked, and quickly replaced by the lighthouse building that still stands today. From there, it heads down King's Cross Road, from where the valley slope can still be seen in the surrounding streets, and into Clerkenwell; the comparatively steep valley is responsible for the viaduct bridges which carry local roads over the valley floor.

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Wikipedia