If you’re in Camden, chances are you’re not far from an underground river.

Ham & High: Fleet Road, Hampstead. Picture: Tom HollandFleet Road, Hampstead. Picture: Tom Holland (Image: Archant)

The Fleet is London’s biggest, and historian Tom Holland fulfilled a long-held ambition when he traced its path through the borough in Febuary.

It runs down from Hampstead Heath, where you can see it, all the way down to the Regent’s Canal, St Pancras and the Holborn Viaduct, where you can’t.

Although the majority of the river has vanished from the surface and you can no longer track much of it above ground (unless you’re lucky enough to know which drains to gaze through), its path has played a crucial and under-appreciated role in shaping the borough, Tom explained.

Armed with an indispensable guide – Dr Tom Bolton’s book London’s Lost Rivers – he had long wanted to walk the Fleet’s length, and he told this newspaper about the experience.

Ham & High: Holborn Viaduct - which emphasises the river valley carved out by the Fleet. Picture: Tom HollandHolborn Viaduct - which emphasises the river valley carved out by the Fleet. Picture: Tom Holland (Image: Archant)

Tom said: “It’s been a long-term ambition of mine. Living in London, you’re always aware of the psychogeography of it all – the sense of these rivers which bear witness to the history of the city.

“To walk the length of the Fleet is to make contact with the history and geography of the city.

“Of course, the Fleet goes through, and begins in, Hampstead Heath, and there you have a sense of London, as it would have been outside of the boundaries of the City of London throughout the Middle Ages and well into the 18th century.”

The Fleet is fed, as are the also-underground Tyburn and Westbourne, by Whitestone Pond. Then it, or at least the most notable branch of it, flows through the Vale of Health and towards the Hampstead chain of ponds.

Ham & High: Hampstead Pond no.1. Picture: Ken MearsHampstead Pond no.1. Picture: Ken Mears (Image: Archant)

And that’s the last time the river is above ground.

But that doesn’t mean all sign of it disappears. Pointing to Fleet Road in Hampstead, Tom explained: “You can follow its route through Hampstead itself.”

Meanwhile, another stream on the other side of the Heath fills the Highgate ponds before heading down below Kentish Town.

The Hampstead branch of the river then snakes down through Gospel Oak – where it was built over by Victorian estates that have themselves long disappeared – before you find it directly underneath the Regent’s Canal.

Ham & High: Hampstead Pond no.1. Picture: Ken MearsHampstead Pond no.1. Picture: Ken Mears (Image: Archant)

The two branches converge underground in Camden Town, then its kinks and bends lead the river into St Pancras where it passes the St Pancras Hospital and reaches a building it has flowed alongside for an awfully long time indeed – St Pancras Old Church.

The church has a history encompassing The Beatles (they posed there in photos in the ’60s) and, apparently, Roman history.

Tom said: “St Pancras is just in amazing condition – there’s so much there. The grave of Mary Wollstonecraft; the Thomas Hardy tree in the old churchyard.

“Reportedly, it’s the oldest place of Christian worship in London, said to date back to Roman times, but that’s probably nonsense – there’s absolutely no evidence there was a church there then.”

Then the river flows hidden towards King’s Cross, where the towering architecture of the modern station owes a debt to the river.

Tom said: “The curve of the concourse is a curve because it’s following the curve of the old station, of course. But that itself was curved because of the river!”

Historians have long marvelled at London’s propensity to build narrative around Old Father Thames, and Tom said the Fleet and other lost Thames tributaries are no different.

“I think what’s amazing is that rivers inspire stories,” he said. “They provide a focus for them, particularly in London when they are covered up.

“They provide a sense of uncanniness – knowing what’s there but also knowing that you can’t see it. Almost the most exciting moment is when you can just hear the river beneath you.”

At King’s Cross the river follows the path of the modern-day Metropolitan line into central London. It passes Mount Pleasant, the inaugural postal sorting office, and heads southward.

It is tracing the Fleet through its lower reaches, Tom explained, that made the river’s topographical importance most plain.

He said: “It’s only when you do the walk that you realise it all – that you realise you are walking along a river valley.

“It’s most obvious when you’re going under Holborn Viaduct. You suddenly see the rest of the route, and London, differently.”