One of the most intriguing relics of ‘hidden London’ opens to the public this month (https://www.postalmuseum.org/discover/attractions/mail-rail-ride/). It is known as the Mail Rail, a network of underground tunnels located underneath the Royal Mail’s Mount Pleasant sorting office. Here, for nearly a century, mail was transported across the capital, on miniature trains, by workers toiling in eerie, stalactite filled conditions. You can now travel on the Mail Rail as it whizzes beneath the capital.

(Photos: The Postal Museum)

All over London there are other subterranean curiosities. Although some, it has to be said, are so secret, and shrouded in mystery, that it is questionable whether they even exist. Fact or fiction? We leave it to you to decide. Here is just a small selection.

Down Street Station
Formerly a stop on the Piccadilly Line, between Green Park and Hyde Park Corner, Down Street station was closed in 1932, but later played a key part in World War II, providing a safe, bomb-proof headquarters for the Railway Executive Committee, responsible for strategic planning of the nation’s railways.

Charing Cross underground
The disused areas of Charing Cross station have been closed to the public since 1999, but have subsequently had a new lease of life as offbeat movie locations, featuring in Skyfall, Paddington Bear and other films.

Tunnel out of Buckingham Palace
There has long been rumoured to be an underground tunnel, dating back to World War II, through which the Royal Family could discreetly flee if the capital came under attack. But as anyone who revealed its existence would probably get sent to the Tower of London, it has to be filed under ‘unconfirmed’.

Tunnels linking Whitehall departments and the MI6 building in Vauxhall
These, too, remain shrouded in secrecy, but there is certainly a network of underground tunnels linking the key government ministries in the event of an attack from a foreign power.

The River Westbourne
If you have ever been puzzled by the green conduit under which trains pass at Sloane Square station, the explanation is simple – though not obvious. It is a small stretch of the River Westbourne, one of a number of minor tributaries of the Thames which flow below ground level.

Tunnel beneath the Old Bailey
According to urban myth, there is an underground tunnel beneath the Old Bailey, which originally led from Newgate Prison to St Sepulchre church, enabling clergymen to visit condemned prisoners without having to run the gauntlet of baying crowds. If the tunnel does not exist, it is still a good story.

Tunnel beneath Harrods
Half the residents of central London must be close to an underground tunnel of some description, and we are no exception. Last year it was reported that, subject to new development plans going ahead, two disused tunnels beneath Knightsbridge tube station – accessible via a back entrance on the corner of Basil Street – might be re-opened to the public again.

Moles stopped play?
Don’t tell the batsmen, but there are disused railway tunnels under the Nursery End at Lord’s cricket ground. They are owned by developers Rifkind Associates.

London’s hidden underground tunnels will never enjoy the iconic status of Big Ben and Nelson’s column. But their very existence is testimony to the enduring charm of our capital – a city you think you know, but which will always take you by surprise.

• Henry & James is Belgravia and Chelsea’s premier estate agent for residential sales and lettings. Get in touch to discuss your needs (www.henryandjames.co.uk/contact).