For most of its length it is a deep-level tube line. The portion between and opened in 1890 and is the oldest section of deep-level tube line on the Underground network. There were about 252 million passenger journeys recorded in 2011/12 on the Northern line, making it the second-busiest on the Underground. (It was the busiest from 2003 to 2010.) It is unique in having two different routes through central London. Despite its name, it does not serve the northern-most stations on the network, though it does serve the southern-most station, Morden, as well as 16 of the system's 29 stations south of the River Thames. There are 50 stations in total on the line, of which 36 have platforms below ground.