While London's tube had much of its 12 lines and 250 miles of track in place well before the mid 20th century, Rome is still struggling to add its third metro line. The problem is an age-old one: the metro runs deep underground and is deep enough so that the tunnels themselves do not interfere too much with Rome's layers of buried civilisations. The stations and air vents, however, need to come to the surface and, much to the frustration of the construction company, they more often than not strike valuable archaeological areas.