("River fleet" redirects here. For ships or boats which patrol a river, see patrol boat and navy.)("Fleet Ditch" redirects here. For the waterway in Maidenhead, Berkshire, see Maidenhead Waterways.) 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.