London is a port city, and Britain became great because of its navy and ability to trade across the globe. The capital’s wealth is built upon that sea trade. There are reminders of London's maritime history everywhere. In Trafalgar Square, we commemorate the 1805 victory (and the death) of Admiral Lord Nelson at the Battle of Trafalgar. At Tower Hill, we remember the 36,000 merchant mariners lost at sea with no known grave. And the Museum of London Docklands examines the triangular trade in goods, sugar and slaves, upon which so much of the city’s wealth was built.

Tourist guides are normally concerned with what is above ground, but London also has a fascinating underground story. As well as the famous buildings you can see on the surface – Westminster Abbey, Saint Paul’s Cathedral, Buckingham Palace, and the Tower of London – London has a network of underground sewers and rivers that repay close attention.