English: Belsize Park deep shelter (south), Haverstock Hill / Downside Crescent, NW3 Shows the location of 887663.In late 1940, the government decided to build a system of deep (air raid) shelters linked to existing underground stations. Each deep shelter consisted of two parallel tubes that might be used for future express underground lines. Seven such shelters were built at Northern Line stations - Belsize Park, Camden Town, Goodge Street, Stockwell, Clapham North, Clapham Common and Clapham South. Each shelter had two entrances with a “pill box” and ventilation shafts; most of these are still there today (February 2011). Some of the deep shelters are in use for secure storage of documents.