The River Fleet is the largest of London's subterranean rivers. Its two headwaters are two streams on Hampstead Heath; each is now dammed into a series of ponds made in the 18th century, the Hampstead Ponds and the Highgate Ponds. At the south edge of Hampstead Heath these two streams flow underground as sewers which join in Camden Town. From the ponds the water flows underground for 4 miles (6.4 km) to join the River Thames.

The higher reaches of this flow were known as the Holbourne (or Oldbourne), whence Holborn derived its name.

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, join in Camden Town and pass under Kentish Town and King's Cross. 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 river then flows down Farringdon Road and Farringdon Street, and joins the Thames beneath Blackfriars Bridge.