London has the largest and most definitely the oldest subway system in the world. It has evolved since the 1860s, and during the course of that evolution some stations were abandoned - or as the British say, disused. To abandon an underground station, all the authorities need do is to close the surface entries and turn out the lights as they leave the station for the last time. Hywel Williams's Disused Stations on the London Underground site is a work of love by a disused station expert. Williams has been to most, if not all, of these stations and he provides full histories and fascinating photos. Be sure to check the introductions to the deep and cut-and-cover sections. They're excellent history.