Some of the features of the Saint Petersburg Metro make it stand out amongst others, even those in the former USSR. It is customary to have stations in the centre of a city built very deep, not only to minimise disruption, but also, because of the Cold War threat, they were built to double as bomb shelters, and many old stations do feature provisions such as blast doors and air filters. In most cities, the lines become shallow or even begin to run above ground as they reach the city's outer residential districts. However, this is not the case in Saint Petersburg. The difficult geology means that all but 9 stations are at a deep level. The design and architecture went through numerous phases. The original stations were predominantly of the pylon type, of which there are 15 stations. Also popular was the column layout, and there are 16 such stations in the system.