Tallinn was one of the urban areas with industrial and military significance in north Estonia that during the period of Soviet occupation (1944–1991) underwent extensive Russification of its ethnic composition due to large influx of immigrants from Russia and other parts of the former USSR. Whole new city districts were built where the main intent of the then Soviet authorities was to accommodate Russian-speaking immigrants: Mustamäe, Väike-Õismäe, Pelguranna, and, most notably, Lasnamäe, which in 1980s became, and is to this day, the most populous district of Tallinn.