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.