The northern part of Estonia is rich in oil shale and for this reason a settlement was erected here already before World War II, which grew exponentially into a city in the immediate postwar years when Estonia became part of the USSR. In 1949 a power plant was built here, fuelled by oil shale. Kohlta-Järve consists of different urban cores built in the 1950s and 1960s, each highly industrialized. The city is very diverse ethnically: it contains people of over 40 ethnic groups. Only 21% of the population are ethnic Estonians; most of the rest are Russians. Kohtla-Järve is the fifth-largest city in Estonia.