While deep, the depression was also brief. The interwar period proved a period of recovery for Ballarat with a number of major infrastructure projects well underway including a new sewerage system. In 1930, Ballarat Airport was established. By 1931, Ballarat's economy and population was recovering strongly with further diversification of industry, although in 1936 Geelong displaced it as the state's second largest city. During World War II an expanded Ballarat airport was the base of the RAAF Wireless Air Gunners' School as well as the base for USAAF Liberator bomber squadrons. In 1942, Ballarat became connected to the state electricity grid by a 66,000 kV line. Prior to this, power supply was generated locally.