Bombing was a nightly problem for the citizens of London during the Second World War. German planes flew overhead every night, and bombs fell regularly. Houses weren't safe, and so the people hid out underground.
The safest place to be during a bombardment was inside the city's subway system. One station -Aldwych - was even semi-permanently converted into a large bomb shelter, with toilets and bedding available. People would stay overnight and emerge in the morning, hoping to find their house still standing.