This year marks the 70th Anniversary of the Blitz in 1940. Following a short Christmas 'break' (!) the Germans resumed bombing on the 29th December. It has always been remarked upon that St Paul's survived the onslaught virtually intact, whilst all around it was flattened:
http://www.bbc.co.uk/news/magazine-12016916The article doesn't give much away, but I would imagine there must be an interesting story here as to how the Cathedral staff, firewatchers etc, fought to keep the building from being destroyed. If the bombers had succeeded not only would one of the great buildings of London have been reduced to rubble, but presumably Nelson's tomb and the others surrounding it would have been blown to atoms. We should certainly be grateful that this didn't happen.