Marble Arch subway complex (east) (London)

 tunnel, underpass, invisible

One third of one of the most complicated networks of pedestrian subways anyhwere in the world, with no fewer than 27 individual entrances and exits (some of which are now closed off). This part of the network includes access to the HydePark car park (under the park) and to Marble Arch Undrground station (there's another section to the west, and another - the strangest of all - to the south). The complex was designed to make sure every section of what had been Hyde park (before Park Lane was turned into a dual carriageway, and the large gyratory was built round Marble Arch) remained accessible to the public - hence some of the curious and little-used exits that seem to go to nothing more than traffic islands. With hindsight these subways were probably just a little bit too confusing, and are now due to be (mostly) replaced with surface pedestrian crossings.