The River Fleet is a subterranean river in London, England. It flows through the city and is one of the main tributaries of the River Thames. Historically, the Fleet was an open river, but it was covered over in the 18th century due to pollution and urban development. Today, the River Fleet runs mostly underground, with sections visible only at certain points. It originates near King's Cross and flows southward, eventually joining the Thames near Blackfriars Bridge. The river has a rich history, once serving as a vital waterway for trade and transport in medieval London.