London is Europe’s largest city, and is a leading cultural, business and financial centre. The most populous city within the European Union, it has an official population of around eight million people. An international transport hub and major tourist destination, for centuries it has been one of the most powerful cities in the world.

Above ground, London allows the eager visitor to feast upon a cornucopia of touristic treats. The city is a network of world heritage sites and towering landmarks such as Tower Bridge, the Tower of London, Westminster Abbey and the Houses of Parliament. The British Museum, the National Gallery and the Victoria and Albert Museum are among a host of cultural and artistic attractions.

Yet another world lurks beneath the city’s congested roads, rainy streets and grey pavements. In this instalment of ‘Cities of the Underworld’, Eric Geller reveals a hidden warren of Roman baths, secret crypts, lost rivers and indestructible bunkers. Eric discovers the subterranean secrets found below all of London’s most famous landmarks.

The majority of people believe that London only has one river: The Thames. But a number of tributaries that were once open streams and rivers are now hidden beneath the city streets, diverted into sewers, drains and culverts. The Fleet – which gave its name to Fleet Street in EC4 - is the most famous of these ‘lost rivers’. It rises in Hampstead in the north and flows underground through Kentish Town and King's Cross, to join the Thames near Blackfriars Bridge. In mediaeval times the river was central to a number of trades. Tanneries and slaughterhouses lined the banks of the Fleet; the dyes and blood turned the river various shades of red.

As our expert host slowly peels back the layers of time, he reveals Winston Churchill’s famous Cabinet War Rooms. This vast complex of secret bunkers was constructed beneath the streets of London during the Second World War. For the British prime minister and his cabinet, the rooms provided an invaluable refuge from the nightly onslaught of Nazi air raids.