The River Fleet was a part of London life before London was even London. This tributary of the Thames, called the Holburna ("hollow stream") by the Anglo-Saxons, is the largest of London's mysterious lost subterranean rivers. And even before the Anglo-Saxons, the Fleet was a major river used by the Romans.
As London grew during the Middle Ages, the area around Fleet became crowded with industrial and residential buildings, and the river became increasingly choked and polluted. Ironically, the disastrous Great Fire of London gave the ever-growing city a golden opportunity to rethink and rebuild London to better accommodate its skyrocketing population. The Fleet River was no exception. Christopher Wren, famous for designing Saint Paul's Cathedral, tried his hand at re-imagining the river. Modeled after the Great Canal of Venice, Fleet received new stone embankments and was given four decorative bridges—at Bridewell, Fleet Street, Fleet Lane, and Holborn—all high enough to allow the passing of large barges.
Sadly, the barges rarely came, and the under-used canal of Fleet once again became as polluted as the rest of the Thames. The canal-cum-open-sewer became an embarrassment, and was bricked over in phases between the 1730s and 1870s. Incredibly, the Fleet River's history doesn't end there. The quick filling-in of the canal sealed and preserved this piece of history until it was recently uncovered:
"It lay buried for 250 years until Wren's Fleet Street bridge was re-discovered in 1999. Museum curator Simon Thurley studied old maps of the area, and worked in conjunction with the Thames Water authority. Thurley succeeded in finding stones from the western end of Wren's bridge, embedded in brickwork from the 1700's in the Fleet sewer, underneath Ludgate Circus." (Source)