Like so many little bits of history, the ghost sign pictured below is something that is so easily overlooked. It can be found on Carlisle Place, SW1, a quiet street close to London's Victoria Station. But you'll have to look carefully.
This sign, quite high up on the wall of a mansion block, directs people to the nearest bomb shelter, under the pavements of the street itself. One cannot imagine such a shelter would have survived a direct hit, but given how badly the Westminster and Pimlico neighbourhoods suffered during the Blitz, it would have provided a safer alternative to staying in one's own home. The sign's an unlikely survivor of the war - the mansion block it's painted on to is exceptionally well kept and the sign does look as though efforts have previously been made to remove it.
Many of the residential buildings in SW1 have vaults under the pavement. The house I currently live in has three - they have a curved ceiling and were primarily designed for storage, although in some houses and flat conversions nowadays you'll see them converted into extra rooms or even wine cellars. One of the vaults in my own home houses the washing machine, tumble dryer and a freezer. As bomb shelters they would have afforded protection from anything but a direct hit or a large quantity of falling masonry. SW1 also has a distinct lack of green space, with many houses and mansion blocks not having gardens, so the vaults took the place of Anderson shelters.
Carlisle Place is one of those typically contradictory streets often found in London - it's a combination of million pound flats in smart mansion blocks, expensive cars parked outside, and a homeless centre with tramps and beggars congregating nearby. Perhaps during the Blitz rich and poor were similarly huddled together in the vaults, seeking shelter from the Luftwaffe's indiscriminate bombs.