All clear
All clear is the signal, generally given by an air raid siren, which indicates that an air raid or other hazard has finished and that it is safe for civilians to leave their shelters; it is commonly used in radios as well. == United Kingdom == During the First World War, the Metropolitan Police introduced their first air raid warning system in London in 1917, which consisted of police officers detonating maroons (a type of loud firework) and the "all clear" was sounded by Boy Scout buglers.