By early 1917 it appeared that the threat to London from the air was over; the Zeppelin menace was defeated. But on 13 June 1917, German Gotha heavy bombers appeared in the skies over the city, dropping some 72 bombs within a mile of Liverpool Street Station. London's defences were quickly overhauled to face this new threat, with a system of outlying observers, sub-control sectors, barrage balloon aprons and gun rings being developed. Air raid warnings were introduced, as were public air raid shelters, and great swathes of London's population sought shelter for hours on end in the Underground stations. The Gotha bombers were later joined by the massive Staaken 'Giants,' and the raids continued until May 1918. However, the formation of the RAF, along with the British defence system, protected the capital against these attacks and also provided sound foundations for the safeguards that protected the country during the Battle of Britain and the Blitz in World War II.