‘Sewers’, says Peter Ackroyd in London Under, ‘exercise a curious fascination upon otherwise healthy and happy people.’ They figure quite prominently in Ackroyd’s new book, from medieval waste pipes, through Joseph Bazalgette’s labyrinthine marvel (‘the most extensive and wonderful work of modern times,’ said the Observer in 1861), to the Thames Tideway Tunnel, a 32-kilometre-long sewage overflow running from Chiswick to Beckton and scheduled for completion in 2020. We meet rat-catchers, robbers and ‘toshers’, an underground breed who earned their living in the nineteenth century by scavenging in the raw sewage for objects of value.

But London Under doesn’t stop at sewers. There are tunnels and vaults and secret subterranean passages, ancient tracks and Saxon halls that have lain buried beneath the surface for a thousand years. Lost rivers, lost Tube stations, lost souls. Other worlds lurk below London, and Ackroyd revels in