Deep in the bowels of London you can find a vast network of subterranean rivers, the largest of which is the River Fleet.

The Fleet starts its journey as two tributaries that join near Camden Town in the north of the city. It used to be a normal river – but thanks to extensive building works in the 19th Century, it is now funnelled underground.

Today, the Fleet’s waters are fed by a vast labyrinth of tunnels carrying the bulk of the city’s sewage. That means a journey along the Fleet is not for the faint hearted as Dallas Campbell discovers in the clip above, taken from the BBC documentary Britain Beneath Your Feet. At one bottleneck, he finds a monstrous ‘fatberg’ – the congealed mass of thousands of homes’ cooking oil and lard. If left untouched, London’s fatbergs could grow to the size of a Boeing 747, posing a considerable hazard to the city’s residents.

Watch Campbell’s exploration of London’s underground river, and his efforts to break the fatberg apart – a thankless task for many of the city’s sewer workers who spend their days in this subterranean wasteland.

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