(All photos © Philip Lindhurst)

Marooning the Great Northern's former station at Highgate are two pairs of single bore tunnels - Highgate East (139 yards) and the longer Highgate West (332 yards). All four structures are brick-lined with portals fashioned from engineering brick.

The west tunnel's Down bore suffered bomb damage in the Second World War, resulting in its western portal being rebuilt in concrete.

The tunnels are now a bat sanctuary, part of an ecological project set up by the Corporation of London and London Underground. Species in residence include Long-eared, Natterers and Daubenton.