Although two other London Underground lines operate fully underground, the Northern line is unusual in that it is a deep-level tube line that serves the outer suburbs of South London yet there is only one station above ground (Morden tube station) while the rest of this part of the line is deep below ground.
Selvom andre London Underground-baner drivet fuldt underjordiske er Northern line usædvanlig, da den er en dybtliggende tube-bane, der betjener de yderste forstæder i Sydlondon, mens kun én station er i terræm (Morden Station), mens resten på denne del af banen er dybt under jorden.