I have a weird fascination with underground rivers, be they natural, man-made, or just plain awesome. In a recent post about phobias, many people expressed fear of things lurking beneath murky bodies of water; what could be darker, or stranger or scarier than something lurking in a body of water that is itself lurking underground? There are many in the world: here’s one of our favorites.

London’s River Fleet

The largest of the so-called “lost” tributaries of the Thames, the River Fleet was a substantial river back in Roman times, when it measured at least 100 yards at its widest point. By the 13th century, it was considered polluted, and by the 17th century it had become the backyard of much low-income housing, several notorious prisons and a great deal of industry which both polluted its waters further and staunched its flow. Ben Johnson wrote a mock-epic poem in the 17th century called “On the Famous Voyage,” detailing an intrepid journey down an excrement-clogged ditch. It became a canal after the Great Fire of 1666, used by the coastal coal trade, and over the next hundred and fifty years the canal was filled, and what sections of the river that remained were diverted underground and covered over.

Nowadays, glimpses of the Fleet are rare — you can see the mouth of it running constricted beneath Blackfriar’s Bridge, and there are a few sewer grates around the city where, if you put your ear to the ground and listen closely, you can hear it flowing down below. The Fleet has become the province of urban explorers, who’ve given us awesome pictures like this one, from Picasa user Steve:

More subterranean fun to come!